Primarily online games that rely on having a base community should never push the performance envolope of whatever is the standard gaming setup (which i'd say is a 3500mhz [processor, 7600/x800 256mb gfx card right now) at the time of release.
Also I've bought every single Quake game, and ALL have performed better than I ever expected, based on footage/images i've seen pre-release. Judging from what i've read about Megatextures and how video memory efficient they are, i'm thinking this will be no different.