When I built my rig initially I skimped on GPU and Memory. One of my goals was to build a good gaming rig. I quickly found that I should have invested more in the GPU and memory.
Originally I went with a budget 6600, but now I have the 7600GT - which seems to be the sweet spot for price v. performance. AT recommends their latest
Video Card Price Guide. They list some as low as $150AR.
For memory, I started off w/ 1GB, but really noticed a significant performance increase when I went to 2GB. I spent a little more, but well worth the extra $$.
Those would be my recommendations.