snake902 :
If you're looking for budget solutions, I've seen some pretty good performance from the AM3 platform with an AMD Athlon II X4 series CPU. They are very cheap and do some good work. Definitely not optimal if you have some cash to spend though.
Assuming you have to upgrade, do you know what you are looking for? How much are you trying to spend on upgrades?
those could be the budget option, but the problem is that it can easily bottleneck newer more powerful card, which makes upgrading is pretty much useless. it also might struggle at some newer title, older hardware might also bring compatibility issue to the future