They're pretty useless at games; I tried running some games on a computer with an Oxygen-based card at my dad's work and the games ran quite slowly. The prices are also sometimes as high as entire systems, but they're meant more for graphics professionals than gamers.
I am pretty much waiting for the next generation of processors to be released, at which point I will probably go for a new machine.

(I basically need something that can run games well but can also run through floating-point calculations quickly, as mathematica really needs good FPU performance to make implicit 3D graphs)