You won't see much performance difference between the E6600 and the Q6600. All a Q6600 is is two E6600's on the same die. Most software and games don't use four cores yet and the one that do isn't that big a difference. Spend $300 somewhere else.
I agree somewhat as well. Your memory will work fine with the quad. But the increase in performance per dollar spent can probalby be spent elsewhere for the time being since the difference between dual and quad right now is only seen in like divx.
and abt the pc (long story short), i had to build a system 4 a cousin. i added a little cash to his modest budget and bought a midrange pc and a quad core. put my e6600 in his system and took the quad core.