logainofhades :
While I agree that they are nice processors, a 4.0-4.5ghz i5 or i7 would smoke your C2Q. 3570k's, for instance, hit 4.0-4.3, consistently, without any real effort.
Since the 3570k already turbo-boosts to 3.9GHz without overclocking, 4.3GHz is barely a 10% overclock which is hardly worth mentioning IMO: 10% is hardly noticeable outside of benchmarks and unlikely to ever make or break performance in any game or application - if your computer is starting to feel slow at stock clock, 10% is unlikely change that in any meaningful way.
That said, the i5-3570 is about twice as fast as the Q9550 which is usually my minimum threshold to start considering upgrades if other factors do not prompt me to do so first.