Yes, all the new intelMacs can run Windows Vista or XP Pro. They can probably run more things... but those are the drivers that are available.
Depending on the MMO, anything from the Mac Mini up to the Mac Pro, and that includes the portables, could run it. WoW for example runs on my MacBook with Core Duo and GMA950 no problem. But a more stringent game like GuildWars, City of Heroes, Vanguard, LOTRO would require more oomph and so you would want to ignore the MacBook, and Mac Mini.