I've got a Gigabyte 81845GVM-RZ mobo with a 256 MB Hynix DDR266. I just bought a Kingston 512MB DDR333 module (because it costs the same as the 256MB). Which one should I insert into the first or slot 0 to obtain the best performance and/or stability or either one will do? I guess I will be getting only a 266MHZ speed but my ram will increase to 768 MB ?

Pretty much so.

"Which one should I insert into the first or slot 0 to obtain the best performance and/or stability or either one will do?"

Kingston says it doesn't matter:
"Virtually any configuration up to 2GB can be reached using any combination of Kingston's 128, 256, 512MB, and 1GB modules."
Usually the best way is to have the ram detected "by SPD" or similar in the bios Setup.

"I guess I will be getting only a 266MHZ speed..."
true
"..but my ram will increase to 768 MB ?"
If both memory modules are compatible with the mboard and each other, yes.
For Gigabyte 81845GVM-RZ
the module listed on the Kingston web site is KVR333X64C25/512
Is that what you bought?
If it isn't, whether it will work in your mboard is a crap shoot - primarily it has to be compatible it's main chipset.
There is no way of looking up which Hynix module is compatible with your mboard at www.hynix.com - if the module came with the system it's probably fine.

Thanks, Tubesandwires. "For Gigabyte 81845GVM-RZ the module listed on the Kingston web site is KVR333X64C25/512
Is that what you bought? "
Yes, I inserted the Hynix in slot 0 and the Kingston in slot 1 & it worked, giving me 768 MB @ 266 MHZ. Only slight problem, I had to call Microsoft via phone (couldn't get thru their server after repeated tries) to re-activate my Windows Xp Home SP2. Is there a way to bypass this?
By the way I noticed theres DDR & DDR2 etched on my mobo next to the 2 DIMM slots. Does that mean the memory modules can run in DDR2 mode and if yes, how to go about it? Buy 2 same brand, speed, size sticks?

"Only slight problem, I had to call Microsoft via phone (couldn't get thru their server after repeated tries) to re-activate my Windows Xp Home SP2."

Just adding one ram module isn't enough to require re-Activating Windows by itself. When you Activate Windows a mathematical "hash value" is assigned according to which of certain hardware you have on your system.
The network card you use to access the internet is assigned a value that has twice as much "weight" as everything else.
You are allowed to change a certain amount of hardware without having to Activate again, but if you change more than the maximum allowed, you have to re-Activate
"Is there a way to bypass this?"
Not a legal one.
You must have changed other hardware as well as adding the ram module.
for more info see www.howtotell.com and look around.

"By the way I noticed theres DDR & DDR2 etched on my mobo next to the 2 DIMM slots. Does that mean the memory modules can run in DDR2 mode and if yes, how to go about it? Buy 2 same brand, speed, size sticks?"

DDR2 ram has more contacts than DDR ram has, the two types use different voltages, and the notch in the bottom of the module, and the bump in the bottom of the slot it lines up with, are in different places.
http://www.kingston.com/newtech/ddr...

There are mboards that will accept DDR or DDR2 ram, but all the ones's I've seen have separate slots for each type.

Kingston does not list any DDR2 ram for your mboard.

"The network card you use to access the internet is assigned a value that has twice as much "weight" as everything else.
You are allowed to change a certain amount of hardware without having to Activate again, but if you change more than the maximum allowed, you have to re-Activate"
I did the follwing:
1. Removed the Hynix, inserted the new Kingston, launched Windows. Asked to re-activate Windows. Unable to get thru to them. Shut down pc.
2. Inserted both Hynix and Kingston. Launched Windows, again asked to re-activate, unable to get thru, shut down pc.
3. Inserted Hynix only again asked to re-activate, unable to get thru, shut down pc.
4. Inserted both Hynix & Kingston again. Asked to re-activate, called Microsoft via phone, re-activated.
I do not use the onboard Realtek NIC. I disabled it in Bios. I use a Vodafone Moblie Connect USB modem. What category should this be? Wireless, DSL or Cable?
2.

"Which one should I insert into the first or slot 0 to obtain the best performance and/or stability or either one will do?"

It is always best to go from high to low, for stability, but it'll only run as fast as the slowest link, so I'd suggest putting the 512mb in slot one.

"I use a Vodafone Moblie Connect USB modem. What category should this be? Wireless, DSL or Cable?"

It's wireless but other than that I don't know what technology it uses.
You could contact Vodaphone by email or otherwise if you like:
http://www.vodafone.com.au/Personal...

Your problem of not being able to Activate Windows except by phone is probably directly related to it not using an expected type of connection to the internet.

I'm not sure whether the onboard NIC would have been included in the "hash" value when you Activated Windows before, assuming you did that - if it wasn't disabled when Windows was Activated before it might have been, but that being disabled now and adding the Kingston ram module still wouldn't be enough by themselves to require re-Activation. You must have changed other hardware as well.

Which configuration will deliver the best performance? 512MB @ 333 MHZ or 768 MB @ 266 MHZ (512MB @ 333MHZ + 256 MB @ 266 MHZ)or is it depending on type/no. of applications running at the same time?

It's always "..depending on type/no. of applications running at the same time?".
In general, you are usually better off with more ram running at 266 than less at 333. It's only ~66mhz difference in any case.

Well said, and thanks for the advice. Now I'm trying to tweak the TCP parameters and others for my Vodafone/Huawei E220 HSDPA 3.5G USB2 modem to try and get the best download/upload speed. I've tried DSLReports.com, Speedguide.net and Speedtest.net. Any comments?

I use TCP Optimizer for my ADSL connection.
It's default Optimized setting works well without you having to fiddle with trying different settings.
Get it here:
http://www.speedguide.net/downloads...
Choose the type of connection at the top - Cable (modem) is default, or DSL (ADSL), or Dialup; at the bottom select Optimal, Apply Changes; you are then prompted to Reboot.
It can make a Cable or ADSL connection 20 % faster (or more) than the default Windows TCP settings.

It only has to be run once, and it uses no resources. Windows will retain the settings, until the next time you load Windows from scratch.