I've seen a massive amount of people asking this question recently, probably because everyone is all antsy for wow's xpac and wants to upgrade in preparation for it, so let me take the time to answer them ALL at the same time.
None. We're in the middle of another generational tech roll-over for both processors and GPUs. Buying now is effectively a waste of money. Hold your horses, limp on with what you have, and buy after the new gen stuff releases. You'll thank me later for helping you avoid feeling like a fool when you drop $1500 and then have a system half as capable as something that costs $1500 a month later.