Well here's my thoughts.
Going from 2 8800 GTS 512mbs to a GTX 480 has made a massive difference. I honestly didn't expect much given the benchmarks, but the gameplay is a lot smoother. I've come to the realization that 512mb of VRAM just isn't enough anymore, and that in and of itself would give you a nice boost in smooth frames.
That said, I would recommend atleast a GTX 470 or 6870 for an upgrade. 2 GTX 460s would be a great setup too, assuming you have an SLI only board, otherwise 2 6850s.