Are you saying You want to upgrade your processor(core2 quad q8400) to something more powerful? If not what processor you currently have?
If you currently have the Q8400 I'd say that you do not upgrade it as investing on such an old processor is not wise. Also computer components that old are really hard to find and are expensive too. You better get a new system.
If you don't want to get a new system get a q9600 from eBay. That processor is very fast but also costs a lot.