Depending on the motherboard model, the q8400 would probably be the best bet (and the seller isn't out of town). I'm guessing you're on an lga775 system and don't want to have to update the entire pc. Going to a quad core will likely help, maybe you can overclock it some. Again without knowing what motherboard you have it's hard to say.
It also depends on what games you're trying to play. However if you're looking to play new or more recent AAA title games such as gtaV, witcher 2/3, fallout4 etc you may need to save up for a new pc. There's only so much you can update on that system, newer platforms and parts perform a lot better. I wouldn't invest a whole lot more money into that system if you can help it.