Is that assuming overclocking the i5 about 4 ghz?
Nope, at stock the 2500K is straight up nearly double the speed of the Q6600: http://www.anandtech.com/bench/Product/53?vs=288
Depending on specific thermals in your PC, the 2500K will ramp single core speeds to 3.7 GHz on its own.
A lot of people don't realize just how much faster processors have gotten in the past few years - in that timeframe, there's been a fairly significant shift to laptops with lower TDPs, and most games have been GPU-bottlenecked anyway.
Personally, on my main PC, I'm still running a dual-core 2.0 GHz Athlon 64 from 2005 clocked at 3.0 GHz. It's lasted amazingly well over that timeframe, but I'm reaching the point where I can't really justify not upgrading anymore. (Particularly since my job will pay for a PC for myself) I'm torn between picking up a 13" laptop with Intel's 35W Ivy Bridge quad-core, or just building a desktop with whatever high-end desktop chip Intel releases in a couple months.