The questions rather are "Which CPU do you have right now?" and "Are you going to use software that can make good use of four cores?"...

You also didn't really make yourself clear: Do you just want a new CPU or also a new mainboard and so on? If you just want another CPU for your current system there's not much to recommend as there'll probably be just one option (in terms of which type you can use)...

Enjoy your meal
Toast