I NEED HELP!

I got a new motherboard (ASUS K8N-VM) recently from a friend (on-board GPU geforce 6100). The manual for the board says I need at least 300W psu plus more for extra devices, so I go and buy a 380W psu (ANTEC Neo HE psu [model# says NEO HE380]). I used a pair of 512 DDR ram (identical ram sticks) from another computer. And I used a hard drive (300gb Seagate [model# ST3300631A]from another computer loaded with WinXP). Also used some ancient cdrom and floppy drive.
So everything is plugged in tight and I flip the psu switch to on. Motherboard led turns on. I hit the case power on and the cpu fan and psu fan spin for half a second then turn off. Won't boot. I mess around with combinations of master/slave jumpers and IDE cables for the hard drive. (motherboard manual says BLUE ide for motherbard. BLACK for slave/optical. GREY for master hard drive. However, the hard drive manual says BLUE for mobo. k . But it says BLACK for master hard drive. GREY for slave.)
Decide to reset the CMOS with the jumper etc. No boot, fans for half a second.
So I eventually have only the psu, motherboard, and hard drive connected. Turn it on, no boot, just the fans for half a second again. I decide to use another hard drive I have. Must be 5, 6 years old. Turn power on and this time it boots. I didn't have the monitor or keyboard plug hooked up, so I soft kill (hold system power button for 4 seconds until it turns off) the system and plug in monitor and keyboard. Turn system on and no boot. Only fans for half a second.
So OMG help me. The old hard drive will boot alone, but not when I plug in the monitor and keyboard cable?
My questions is, what is wrong? Does this newer hard drive require more power than the older one and this 380W psu isn't sufficient because even with an older hard drive just by adding the keyboard and monitor make the system suck too much juice from the psu?

ATX mboards are always powered in some places, even when Windows is Shutdown or in Standby or Hibernate modes.
You MUST remove the power to your computer power supply when you are fiddling with any connection or ram or card inside your case.
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When you install ram modules that work in another mboard that does not necessarily mean they will work in your mboard.
The ram you install must be compatible with the mboard you install it in, which is determined primarily by which main mboard chipset it has.
In the worst cases, the mboard will not boot at all when certain types of ram modules are installed.
See response 5 in this for more info about ram compatibilty, and places where you can find out what will work in your mboard for sure:
http://www.computing.net/hardware/w...
Correction - Mushkin - www.mushkin.com
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If you're not sure whether your mboard not booting is caused by you using incompatible ram modules, you can remove all the ram and try booting - if there is nothing else wrong, and if the appropriate case speaker is connected, or in some cases an amplified speaker to the appropriate onboard sound port is connected, the mboard will beep in a pattern that indicates no ram has been installed. E.g. with many mboards with an Award bios version, you will get a beep lasting about a second, silence for about a second, a beep for a second, etc., continously. You do not need to have a hard drive connected to do that test.
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You can also have problems with booting if the contacts on the ram modules are getting poor connections.
See response 2 in this - try cleaning the contacts on the modules.
http://www.computing.net/hardware/w...
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The mboard manual probably has all the info you need to connect your data cables correctly. If you are using master and slave jumpering on the hard drives, it doesn't matter what color or which of the two connectors other than the blue one to the mboard you connect your hard drives to. What does matter, if it is even possible to physically get it wrong because of external projections on the cable connectors and slots in the drive and mboard connector shrouds, is the stripe on the one side of the data cable is on the same end of the connector on the drives and the mboard - on the drives the stripe is next to the power connector. If that is backwards on one end, that drive will not work and will not be recognized by the bios.

If you are sure the data cables and jumpers are right but still a drive is not recognized by the bios, take a look at the data cable and make sure that you have not ripped the cable at either of the edges and severed the wires, usually right at a connector or underneath the cable clamp there - that is very easy to do by accident, especially with 80 wire data cables.
Pull on the center of the data cable, not the edge, if it has no pull strap.
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Hard drives draw very little power - that is not an issue. If you look at the power ratings on the label on them you can see that for yourself.

"I used a hard drive (300gb Seagate [model# ST3300631A]from another computer loaded with WinXP)."

If the hardware on the computer that hard drive had Windows setup on is more than a little different from your new mboard and the hardware connected to it, XP will hang right when Windows is supposed to start to load. In that case you must run a XP Repair Setup.
An XP Repair Setup will not harm your existing Windows installation.
You will need a Windows CD of the same version as the one of your Windows installation, and the Product Key, preferably the one that was used to install it, but it can be one for the same version as the one of your Windows installation.

how to do an XP Repair Setup, step by step:
http://www.windowsreinstall.com/win...