the 6600's also have smartshader 3.0 for better picture quality
ATI used to have the better cards last generation, this time it's Nvidia's turn. when ATI releases their R520-based card, they might come out on top again. but for now, the 6600 GT is about where to go.
I'm an AMD fan at the moment, and I can say the Athlon 64 X2's are really powerful.
the 3800+ is for $376 at newegg. I know it's not as low-cost as the ones suggested, but benchmarks put it ahead of the Intel's even in multimedia most of the time.
although the MSI K8N Neo4-F boards are really good, and yet inexpensive. $83 from newegg.
stay away from Chaintech video cards, a lot of people have had them die.
they have the eVGA 6600 GT's for $153 (PCI-E). I'd recommend one of those.
from the egg (newegg) you can get 2x512MB PC3200 RAM from PQI, for $117.31
this RAM is pretty good. good overclocker too (if you so choose) and it's not expensive.
instead of getting a seperate sound card, you can use the onboard sound of the MSI board. it is actually 7.1 (8 channel) sound, which is more than most sound cards. the quality is good too.