As others have clarified, the Q9300 is superior to the Q6600 at stock speeds: the clockspeed is a teeny bit faster, the cpu architecture more advanced (Penryn 45nm), greater power efficiency. On the other hand it typically costs about 40 per cent more.
There are many Q6600 overclockers out there. It will probably be remembered as one of the great overclocking chips of the x86 era. If you're going to spend the money on qualite enthusiast kit, why not get a good heatsink and overclock the Q6600 to 3+ GHz? It's simpler on the Q6600 for a variety of reasons (higher multiplier, lower default FSB, more mature production).
Also, you probably won't require 750 W from your power supply. What you've described so far will run fine on 500 (even less if you aren't overclocking). A qualite manufacturer is more important than the theoretical wattage total.