Probably not MMOs though, remember that they are geared towards targetting a fan base that doesn't necessarily have good hardware to play today's games, that's one of their appeals.
I'd go with the Q6600 though, it's pretty pointless to base your decisions on MMOs since they are not really very demanding at all (today's top MMO can play on high/maxed on 4 year old hardware) and when quad cores will actually be useful then you'll have it already.
The easy answer is the Q6600. Especially if you are talking about keeping this thing for 4 - 5 years, which is impressive in my mind...i have had my X1900XTX for alil over a year and a half and other than my hard drives it is the longest I have ever had a computer part since I was in the 7th grade...lol. But that aside, Im pretty sure that WoW takes use of dual core as my roommate saw a big boost once he upgraded, but that could be do to offloading the OS. But who knows...if your situation changes and in the next 4 - 5 years you upgrade to vista I then you will have wished you went with the quad and games will definitely take advantage of it soon so I strangely urge you to go with the Q6600 and if anyone asks you the same question from now on always say the Q6600. If you are going to pay over $200 for a CPU then it should be a quad at this point if its under $200 then obviously dual core, but budget should be the only thing stopping people from buying the quad. hope this helps! Oh and yes...even at stock speeds I would still recommend it.)