I'll recommend you getting this...
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145194
CORSAIR XMS2 DHX 4GB (2 x 2GB) ($75.00 after $30.00 Mail-In-Rebate)
Corsair made their reputation in the enthusiast community as high quality, both in terms of performance and reliability.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150287
XFX PVT98FYDF9 GeForce 9800 GTX(G92) 512MB
($169.99 after $20.00 Mail-In- Rebate
4870 is way oexpensive If you wait couple of month you can get it for $169 When I first got 3870 back in Feb for Crysis, I've paid $279 today they are $129 it's not even 6month yet
Antec Twelve Hundred over Nine Hundred (For future upgrade and wire managment and better cooling)
PC Power & Cooling S61EPS 610W ($89.99 after $20.00 Mail-In-rebate)
The best power supply goes to PC Power & Cooling and Antec
Instead of buying 8GB memory or additional hard drive, why don't you buy games.
-Good Luck-