Depends on the game - Everquest is possibly the worst coding mess ever imagined, so you need a massive spec to run well everywhere in a raid situation. WoW runs nicely on almost anything with it's low polygon models, Eve likes a reasonably powerful card to render its quite frankly stunning visual effects.

As a general rule, most MMO games have some kind of customisable avatar, so plenty of memory (video and system) will help prevent the game having to go to hard disk constantly to load textures. I would also put forward the hypothesis that as MMO games are updated / expanded frequently there *may* be meaningful multiple CPU support encoded earlier than stand-alone games, but then who doesn't include at least a dual-core in a new rig?

Last tow posts took both my examples...although some MMO's do like better systems (Age of Conan), a steady internet connection is far more important.

Quick question (sorry to hijack) for the EVE guys: Since the graphics pack, have the major gameplay issues (Read, the entire Armarr race) been fixed yet?