best value card for gaming right now is 7900 GT for $300. As for mobo and CPU, you might want to wait for the new AMD socket platform which is going to be available starting tomorrow, although it might be more expensive than the current socket 939 platform CPUs and motherboards. Also the new platform will not be significantly faster, so it's ok to stick with socket 939. So i'd recommend getting a low end CPU and overclocking it, such as Athlon 64 3000+, but if you are not overclocking you can just get a higher end one such as 3500+. Keep in mind you can also get a dual core Athlon 64 X2 but it doesn't currently help in most games (but it does help multitasking on your desktop apps). Also you can go for a socket 939 opteron such as opteron 144 (single core) or opteron 165 (dual core) which cost a little more than Athlon 64 but have larger cache and overclock better. As for motherboard, I would go with nForce4 based motherboard, and if you want to overclock get a DFI (such as DFI's budget board NF4 infinity or the higher end NF4 Ultra-D). If you want to keep SLI as a future option (dual graphics cards) get an SLI board, but they usually cost more.
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