The OP should check out the folding rig recommended this month on Bit tech:
http://www.bit-tech.net/hardware/buyers-guide/2010/05/07/pc-hardware-buyers-guide-may-2010/6
It seems that any dual core should be fine if you are using folding clients which use graphics cards. Hence either one of the 45W Athlon II X2 or E5000 series processors would seem to be a good choice as they have relatively low power consumption.
TBH,I would go with at least a G92 based card since they are supposed to support a newer version of CUDA.
Also the 8800GTX will consume far more power than a bus powered 9800GT. The GTS250 gains 12 more stream processors and higher clockspeeds when compared to a bus powered 9800GT.