The only difference between a soft quado and the real thing is the way anti aliased lines are implemented. Aside from that they are the same(ie no difference in performance/stability).

Before going down the ATi route (which are indeed excellent cards for games) I'd check out some of the other forums for reports on how the FireGLx/z1 drivers are behaving... there were a great deal of reports on instability and artifacting in maya with the earlier drivers, and unless ATi have fixed them I'd steer clear imo.