Here's the current PC I'm trying to put together:
Asus A8N SLI Premium mobo
AMD 3500+ 2.2Ghz 64bit
Western Digital 200GB SATA HD
Asus N6600 128MB GeForce
512MB DDR 3200
Plextor DVD+/-R/RW
winXP

Some time earlier, I had posted about a PSU that wasn't working on this PC - an Aspire ATX-AS520W to be exact. Now I have since replaced that PSU with an Antec 550W truepower and am having the same issues I had with the Aspire. When the 12v plug isn't connected, all the hardware components fire up fine - except for 2 fans, which I can fix. But when the 12V plug is connected, the only thing that functions are the red and green LEDs on the mobo.
Aspire Specs:
520W
+3.3V = 30A
+5V = 29A
+12V1 = 16A
+12V2 = 18A
-5v = 0.3A
-12V = 0.8A

Antec Specs:
550W
+3.3V = 32A
+5V = 40A
+12V1 = 19A
+12V2 = 19A
-12V = 1.0A
+5Vsb = 2.0A

Help me! I need this PC to work but don't know what to do from here. Do i have a dead mobo, can the 12V plug be replaced? I'm open for suggestions.

Obviously your problem probably wasn't the power supply.
If you sell one, keep the Antec.

It could be the mboard, but that's unlikely.

Are you sure the ram you got is actually compatible with your mboard? - if it isn't it can cause all sorts of problems, including no boot.
This is apparently a major problem with DDR ram especially - going by the PCIxxxx rating, etc. isn't enough.
You can go to the web site of major manufacturers and distributors and look up what works in your mboard for sure, using your mboard make and model, or by having them scan your working mboard.
e.g.
www.kingston.com
www.crucial.com
www.memorygiant.com

If you have brand name ram, you can often also look up which mboards the modules will work in using the ID string of the ram.

Are you sure you installed the cpu properly and it is seated properly?
Are you sure you have installed all wiring to the mboard properly?

Are you sure the cpu you installed is supported by your mboard? There are cpu support lists for each model at most mboard web sites.

Is your bios version new enough? This can be hard to determine unless the mboard is working. How new it is depends on how long it has been sitting on a shelf somewhere since it was made, not when you bought it.

Those cpu lists often state which bios version you must have to support the cpu, or the whole list is for the newest bios version.
If you installed a cpu the bios version on your mboard doesn't recognize, the mboard will either start up but set the cpu to default values and display a message that you need to update the bios, or the mboard will not boot at all.
If the latter, you, or someone can do it for you, must install a cpu the bios version you have DOES recognize, flash the bios to a newer or the newest version, THEN you can install your cpu and the mboard should boot fine.

By the way - look in your mboard manual - there may be a list of what the combos of colors of the led's mean, and that may give you some good clues. If that info isn't in the manual, look on the mboard's web site for info about that.

I cannot do anything with the bios because I can't get the board to go to post. All my parts I had purchased from Newegg and the Premium SLI mobo is one of the latest boards. Unfortunately, it seems that it is also one of the most problematic. The mem is Corsair Valueselect DDR 400 PC 3200. According to the Corsair homepage, with its compatibility lists, it is compatible to this board so that shouldn't be the issue.

The LEDs mean nothing. One lights if no power is going to the SLI slot - which I'm not using so it does not require a connection there with a single card. The other is the green Power LED - same as any other board, under the card slots.

"I cannot do anything with the bios because I can't get the board to go to post."

You have stated that the mboard does not boot several times in this and your other post - of course you can't get into the bios.

"......Premium SLI mobo is one of the latest boards."

See the bios info below - it's been out for a while.

"According to the Corsair homepage, with its compatibility lists, it is compatible to this board so that shouldn't be the issue."

If it's on the list, that's good.
Are you sure you have it in the right slots - sometimes certain types must be in certain slots - see your manual.

"The LEDs mean nothing. One lights if no power is going to the SLI slot - which I'm not using so it does not require a connection there with a single card. The other is the green Power LED - same as any other board, under the card slots."

Are those guesses or did you confirm that info somewhere? Not all mboards have led's, and what they indicate varies widely.

This may not help, but to take the drives out of the mix, disconnect the CD/DVD and the hard drive data cable(s), try booting, with the extra power connector from the PS connected of course.

This may or may not work:
Your mboard has a feature called Asus Post Reporter (page 61 of the PDF manual) - it's dependant on the Speech Reporter IC setting in the bios Setup - it is supposed to vocally tell you what is wrong with your post. If it is Enabled in the bios by default (it may not be - your manual does not say specifically but it appears the default is Enabled).
You must have amplified speakers connected to the proper port on your mboard - see the manual.
......

Look up the cpu support list for SLI Premium here
http://support.asus.com/cpusupport/cpusupport.aspx

You will see there are many different minimum bios versions you must have to support 3500+ depending on which version it is.
(By the way, if you are trying to use a version of the 3500+ meant for mobile applications, it is frequently the case a desktop mboard will not support a mobile cpu - there is a list of supported cpu core voltages in the manual - your cpu must use one or more of them - but the bios must also be able to recognize the cpu code.)
As I said before, if the bios version you have on the mboard is not new enough (not a high enough version number) it is quite possible your mboard will not boot until you install a cpu your bios DOES recognize, you update the bios, THEN you can install your cpu and the mboard should boot fine.
You can figure out what cpu your bios is more likely to recognize from the list of supported cpu's - the lower the bios version number required, the more likely.

The list of bios versions your mboard could have are listed here:
http://support.asus.com/download/download.aspx?SLanguage=en-us&model=A8N-SLI%20Premium
Depending on how long your mboard has been sitting around since it was manufactured, it could have any of those versions, except, probably, the newest one.
Determining which one your mboard has is often difficult or impossible to determine if it is not working - you could try looking at the mboard packaging, it's box (date or bios version labels?) , and on the mboard itself (a label on or near bios chip?) for indications of when it was manufactured or what the bios version is, but often there is nothing like that.
If you do find a date or a bios version, compare that to the same on the bios versions listings.

Yes, I did confirm the LEDs with Asus support when I tried to put the PC together with the aspire PSU. At that time, their first response was for me to change PSUs, so I did. Don't assume ignorance, I've been working on this PC for quite some time now and calling every support number I could think of when I had problems. I will attempt to run post with no hardware connected to the board, as I will likely be removing it if that does not work.

Re: Asus Post Reporter
and
"You must have amplified speakers connected to the proper port on your mboard - see the manual."
This is what is required for two other recent Asus mboards in posts here recently. No harm in also connecting a mboard skpr as well, just in case.
.....

"Don't assume ignorance, I've been working on this PC for quite some time now and calling every support number I could think of when I had problems."

I merely asked. You could have mentioned you got the info from Asus. I didn't see anything about that in the manual, so I wanted to know where you got the info.

You have also answered why you got the second PS. I would have advised you to try a borrowed one or one from another computer you have if you could, as new PS's are not often defective.

We get posts from all sorts - novices to experts. We can't respond properly if not enough information was provided to us, and most people make too many assumptions.
e.g.
- we have been getting a lot of posts lately about someone building a new system with DDR ram and getting no boot, or adding DDR ram to a previously working system and getting no boot, and many turned out to have ram that is not compatible with their mboard - some got back to us saying the compatible ram works fine.
- I built a new computer with new pieces a couple weeks ago that had a bios version that was not new enough for the Sempron cpu I was using (an MSI mboard, ATI chipset - ATI RS480, a.k.a. MS-7145 ver. 1.4) - in that case, the mboard booted anyway, the cpu was using defaults, and it displayed a message that I needed to update the bios, but it's just as likely, or more likely, the mboard will not boot at all in that situation.