Depending on your need, but I would say if you are upgrading CPU now, the best you could get is a Z77 Chipset but you need to upgrade the mobo if you don't have that. I would say upgrade CPU so you could make way for better GPU to upgrade in the future. Something like a Sandy Bridge processor or even Ivy Bridge if you like depending on your chipset.
what are you wanting to do with the computer? if its gaming then upgrade to a dedicated videocard and to 8gb of ram. after that then look into a better cpu.