Current computer spec

Processor: Intel Pentium 4 2.4ghz
Memory: 1024 MB RAM
Video Card: Radeon 9800 Pro 128 MB
Sound Card: RealTek AC97 Audio ( I do not plan on upgrading this ATM )
Windows XP Professional

Now I can run games such as battlefield 2142(low settings), company of heros(low and high settings), and C@C3 Tiberium Wars( high and medium), Planetside ( I have FPS problems when it comes to big red alert battles). But later in these games FPS just sucks and i've done speed tests and my processor gets rated 2 out of 6 and this is just unacceptable for future games that I am looking forward to such as Crysis and Starcraft 2.

I would like to spend no more than 600 dollars and this is what I have found so far.

http://www.smksuperstore.com/catalog...m.asp?ID=18546

I want this combo but I do not know if the motherboard is too weak or not so I do not know if I should spend the money to upgrade it. If I do not need to I was thinking about getting 2 Atlas Precision GB of memory and not upgrading my video card to a GeForce 7600 GT 256MB. Or should I get 1 GB of memory and upgrade my video card?

http://www.partspc.com/Recommend.asp...ageField3.y=11

With this combo the motherboard looks better but I don't know if any of those video cards are better than mine or if I should just keep mine and stick with the 2GB of ram. This combo is also a lot cheaper.

Again whats stumping me is the motherboard in these combos, and if its better to get 1GB of memory and a new video card or just keep my card and get 2GB of memory, and whats the difference between the memory advertised on both sites? I'm trying to not go over $600, thanks everyone.