Depends on what you're doing.
It really would help to know all the specs of your current system and what you intend to be using it for. It's possible that what would help most is a RAM or video card upgrade.
I'm not trying to claim that I know better than you which part of your system needs upgrading. I'm just saying that until we know more, any recommendation that I or anyone else gives would be a stab in the dark.
The E8400 is at a good price point, but if what you're doing is heavily multi-threaded you'd probably be a little better off with a Q6600. If you've got an E6600 right now, it's possible that what you'd really want to do is upgrade your 7600GS to an HD4850. Know what I mean?