Be sure to consider the following aspects when going for an upgrade:
GFX wise, there are 2 main roads to go:
ATi or Nvidia: Crossfire Or SLI
SLI and Crossfire are kinda the same thing, but different technology. It allows for 2 cards to be used at the same time for maximum performance.
If you want to go SLI, you need a SLI RDY system, be sure to check
SLI Zone for more details.
More info on Ati Crossfire here:
ATI CrossFire - Wikipedia
And on SLI here:
Scalable Link Interface - Wikipedia
It all depends on what kind of setup and hardware you prefer...