It really depends on which MMORPGs you're talking about. WoW has very, very low requirements and a 9800GT is more than enough power for it. Heck, I played with everything maxed on a 6800GT OC and everything was smooth at 1680x1050.
On the other hand, EVERQUEST 2 is a game that will eat up all your ram and cpu. By design, EQ2 is more cpu intensive, and a multicore will really increase your fps. (especially since a recent patch included multicore support). Also, get at least 4GB ram and at least a 4850/9800GTX+.
I haven't played any of the newer MMOs like Age of Conan or Warhammer, but they are likely to be gpu intensive, so a minimum 4850/9800GTX+, and once again, at least 4GB ram.
Regardless of which game though, a good rule of thumb is to just get the best gpu that you can possibly afford.