Meh, a Q6600 is still a VERY good chip, theres better no doubt but 98% of gaming isn't about the CPU, and a quad core from when single cores were still barely being taxed in gaming is going to last embarassingly(and boringly when you like to upgrade) well
In terms of gaming now, always, get the best GPU money can buy/worth the value. IE 2x6870's are faster and cheaper than a single 580gtx, basically get the best your money can buy, and put as much money as possible forwards for the gpu's over other parts as you will see by FAR the biggest performance boost come from gpu's.
Lets say you had £300 to spend today and currently had a 4870/260gtx or something, you could spend £150 on 8gb mem, a 2500k and a mobo, or you could buy anything from a single 6850, to a single 6950 or dual 6870's for £300 and have a FAR faster gaming rig with the old cpu and the new gpu(s).
Is a 2600k worth it, for gaming, not even slightly, there will be few if any games faster on a 2600k than a 2500k, and you're talking 1-2%. The saving is more than worth it, even more so if that extra circa £100 went on gpu's than cpu.
The Q6600 at stock, isn't good enough, at 3.2Ghz, will pass the limit on most old single threaded games, and most newer multithreaded games, if you could hit 3.6-3.7Ghz there would be very very few games that would require more CPU power.
Basically ignore the argueing, give us a clue on how much you can spend, and when you want to upgrade.
For instance, if Bulldozer is good it will either offer an alternative and potentially great value or even better performance, or will at least probably cause some price drops on Intel. If you could wait till March/April, Intel's Ivybridge chips will come out, nothing fancy by themselves but could tank 2500k pricing and 2600k price would certainly drop, probably to within £20 of the 2500k, at which point the price difference makes the 2600k a great choice.
Theres loads of options, personally with a 5850 I'd go with GPU first, I'd maybe try to eek out a few more Mhz from the cpu, and I'd wait a while to see how bulldozer and pricing turns out. Even on the gpu, basically within the next month we should get a lot of great info on the new HD7000 series cards, from expected dates, prices, performance, again you'll looking at, on release, huge cut prices on hd6000 series stuff, potentially awesome hd7000 series cards and just flat out better value for money.
In 2 months theres just no question, everything will be better value due to stuff coming out gpu and cpu. AFter that, rather than go for the top line cpu and highest GPU, go for the next to top gpu/cpu, save a bundle of cash, and you won't notice the difference. 2500k/7950, or a FX8120(cheapest octo core)/7950 is likely to be a ridiculously awesome value computer with great performance in the next 1-2 months.