Low or mid range dual core sounds good, like the e6600. There isn't much use for a quadcore yet and by the time there is they'll be so much cheaper that you can just buy one then and still save money as opposed to getting a q6600 now. For example, the Q6600 is around $530 now and the e6600 is around $230. So you save $300 getting an e6600. And the Q6600 is supposed to drop to $266 in Q3, so if that pans out you could get one then and have both a e6600 and a Q6600 for less than a Q6600 now.
Of course, I doubt a quadcore will be a big help then either, but the point stands. Maybe replace with a quadcore halfway through next year or something.