For RAM, you should get your RAM to run in dual channel. So get 2x512 MB matching sticks, and sell your 256 stick away.
For graphic card, i would suggest some card like 7600 gt, 7800 gt or if you have money, 7900 gt. That might cause you to buy another mobo though.
For CPU, go for AMD. If you OC, then get something like an Opty 144. If you dont, then get an 3700+, it is awesome. If you have cash, i would recommend go for a 165 Opty, it OC like ****.
DFI Lan party UT NF4 Ultra
AMD Opteron 146 2.0 Mhz @ 2.8 GHz
OCZ Gold Edition 2x512 MB DDR 500 timing 2.5-3-3-8 @ DDR 560
XFX GeForce 7600 GT 256 Mb GDDR3
Antec Lanboy Mini Tower Case
Antec True Control 550W ATX2.0 PSU
Maxtor 300 GB 7200 RMP SATA 16Mb Cache
17\'\' TFT Screen 12 ms