I have a Dell XPS 420 Dual 2 Quad Q6600 2.40 GHZ 2.39 GHZ. I want to put a more powerful processor in it for gaming.
Should I change it to the 9000 series processor or the Core i7 processors? If so will a new mother board fit inside of the case?
Probably, but why change? The i7 doesn't offer that much performance gain over your q6600; will cost you at least $500 including ddr3. I would upgrade the video card instead to the 4770 and try it with your dell power supply. It may work fine.