Bang for the buck processor? Easy. The AMD Athlon II X4 620. It is a full quadcore, has most of the performance of the higher end quadcores, overclocks very well, is on a socket with a decent upgrade path, and only costs $100. This puppy won't bottleneck 2 5970s in crossfire when overclocked!
You can't beat that, at all.
If budgets are even more of a constraint then the Athlon II X2 240/250 for $50 can't be beat with its good dualcore performance and great overclocking.
OP's talking about Bang for buck LGA 775 processor.