Intel e8400: very fast with good price
 

Like GF8800GT G92 512mb 700mhz, the processor e8400 (2.4 with 6mb cash) is very fast and worth the price, another sugestion is a fresh and ligth Win XP without too much updates and installed programs.

Now, if only BoB would behave better on multithreading... Then even the old Q6600 would be a good choice.
On the other hand, here in Eastern Europe it's impossible to find C2D8400; only 8200, and even that is a little scarce.