I have a Q9450 and I like it a lot. However, what will you be doing with your computer?
If it is primarily for gaming, then I recommend the E8400 which runs at 3.0GHz. Nearly all games right now that "uses" multi-core processors are not optimized yet. It will take a while longer for games to be optimized for Quad Cores.
If you do multimedia stuff (specifically video), then by all means buy the Q9450.
DDR3 is too expensive for the little performance boost you get. Unless you care about synthetic benchmark numbers, you will not notice any performance difference. Want more performance dump DDR3 for DDR2 and use that money for a faster CPU.