An overclocked Core i7 920 on its own is a fairly good folding chip. With $300 and that kind of rig I'd get 3x2GB cheap RAM and perhaps a GTX 560 Ti.
I'm not sure how well 3x1GB would do if you want to fold bigadv units, but if it's just regular SMP, 3GB would be fine.
I wouldn't recommend lower than a GTX 550 Ti for Folding because low-end GPUs will rob some CPU performance from you and will not be worth their power consumption. Note if you can net a really cheap GPU which consumes very little power (<10W), you could just get components to OC your CPU as high as it can go and not fold on the GPU. I kinda do that with my home server which has no graphics at all and is accessible through SSH only.
__________________  "The computer programmer says they should drive the car around the block and see if the tire fixes itself." [src] |