can you find the link of your computer?
there should be like a sticker that tells your series number, just type that in, and give us the link.
I dont think you need 4gb, its nice to have. WoW dont need that much ram, and the only game i see that needs more than 2gb of ram is just Crysis, i can run other fps normally on the highest setting.
Is this your motherboard?
if it is, then the 8800GT will be great, or you can go a little more and get a better one like the 4850 for about $35 more, or even one that outperforms them all for about $150 more. (more or less depending on which one you prefer)
Get a new PSU, from what i seen with the prebuilt computers (unless its made for high end gaming) they only come with PSU with like 380W the most, and the 8800GT needs about 400W, but i suggest getting a quality one (made by Corsair, Antec, OCZ to list a few) with 450W just to be on the safe side
You can get the OCZ 500W for $60
OCZ StealthXStream OCZ500SXS 500W ATX12V / EPS12V Power Supply 100 - 240 V
Then the 8800GT
EVGA 512-P3-N801-A3 GeForce 8800 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card
For $50 more the 9800GTX+
EVGA 512-P3-N873-AR GeForce 9800 GTX+ 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card
I wouldnt go with the Nvidia 200 series, i think they are overpriced...
Or if you are more of the ATI type of guy
HIS Hightech H485F512P Radeon HD 4850 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card
For $100 more, you can get the 4870
SAPPHIRE 100243L Radeon HD 4870 512MB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card
From whats going, it seems that the 4870 is the best bang for your buck.
so pretty much the max you will spend on a graphics card and PSU would be like $370.
EDIT - it seems like you need to go with more ram since you only have about a 1GB. I dont know if you want to get some new ram with faster speed, or just buy another 2gb of ram and add it, and choose the same speed of ram and save about $10-$20. Or you can get the faster speed of ram, but your computer will go with the slowest one, which if you have the 667mhz speed, and got some 800mzh, then you will only go on 667mhz, and probably wont go Dual Channel.
Unless you buy a new mobo and OC, but i wouldnt recommend to OC if you never done it before and have no one to help you.
Unless you got the RAM thats 800mhz, which i dont see often... I only see 667mhz on prebuilt computers.
You should see how much stick of ram is in your computer, if its 2 x 512, then you can buy 2 x 1gb of ram and dual channel it.
or if its 1gb, then you can either get another 1gb and dual channel, or you can just have 3gb and not have dual channel.
Again, CPU-Z can tell how many sticks you have if you dont feel like opening your computer.
whew... done with this essay, time to sleep.
any more questions, feel free to ask, someone will help you out, or if this thread dies out and you still have questions, just PM one of us.