It is worth looking at what Intel has to offer,but if you intend to spend well over £100 on a CPU,I would say wait for Piledriver and Trinity to be released. This should happen around Q2 or Q3 this year. Your Q6600 is still not too bad especially if you can overclock it a bit more.
Your PSU is a TX650W I assume?? This should be fine with many graphics cards ATM.
OK,lets have a look at some cheaper AMD CPU options.
This CPU is very good value
IMHO:
http://www.scan.co.uk/products/amd-p...che-95w-retail
It has a 95W TDP but has locked multipliers. I suspect you should still get a decent overclock from it.
If you want a cheaper stop gap CPU and want some fun overclocking I would consider the following CPUs:
http://www.scan.co.uk/products/amd-p...che-95w-retail
http://www.scan.co.uk/products/10-ca...200mhz-95w-ret
The first is based on a Phenom II X6 and has the potential to be unlocked to one. It also has an unlocked multiplier.
The second has a £10 cashback which you can get from AMD. It is known to be good for overclocking even on a stock cooler. For just under £80 it might be good fun to tweak.
However,remember the FX CPUs have lower single thread IPC compared to a Phenom II X4 but can overclock more.
Of course there is the good old Athlon II X3:
http://www.scan.co.uk/products/amd-a...che-95w-retail
There is a chance of unlocking it to an Athlon II X4 or Phenom II X4. One of my mates got a recent one upto 4GHZ quite easily.
Regarding the motherboard,do you really need a Crossfire capable one??
I know friends who have both these motherboards:
http://www.scan.co.uk/products/asus-...-%28x16%29-atx
http://www.scan.co.uk/products/gigab...-%28x16%29-atx
They are both excellent for overclocking.