If your on a budget, and you want quad core, my suggestion is to wait when quad core isn't still a relitively new technology. With that in mind, if your computer is super slow and you need an upgrade now, AMD Athlon 64 X2's are cheap, and will run just about any game today and still have some power in reserve.
If you want to spend thousands now, so that you don't need to upgrade for 5 years then go ahead, buy a quad core processor, make sure you do your own research though in finding the best processor, look at benchmark reviews on specific processors and pick one that works the best. But don't just go by benchmark reviews, also check to see if they have any major flaws. Intel's first dual core processors had problems and intel didn't seem to care, they put it on the market anyways ahead of schedule.