Your q6600 is still an ok processor. If you are just trying to game on it, I would get a new graphics card like a radeon 7850. If you have an app that needs to do a lot of number crunching, then you'll have to upgrade the cpu, which means you need to replace ram, motherboard, and probably power supply as well.
What Flexxar said is totally right, graphics might be the thing to invest in if you are gaming.
£200 is a slightly awkward budget for trying to upgrade CPU because you'll definitely need a new motherboard+ram, PSU is questionable too. It's also worth assuming that if you have a good quad core already, you probably don't want to step down to a dual core. This is where you hit price problems, as you can get a solid quad core AMD setup for that price but you will struggle on Intel.
If you really want to get a solid performance upgrade and are sure that you are CPU limited, I would personally try to stretch the budget to say £240-250 at which point you can afford a 3rd gen i5 Processor, a cheaper motherboard (H77 probably) and some ram. In theory that wouldn't require a new PSU.
What performance are you trying to improve? Do you plan on gaming, or are you working with some heavily threaded applications?
If you want the overall system to load things quickly and you want to multitask, then you should get an ssd and/or more RAM.
If you plan on gaming then you should get a new graphics card.
What are the other components in your system?
As others have mentioned, I too would not recommend that you upgrade your CPU as you would have to upgrade your motherboard, and your RAM to DDR3.
Hope this helped!
November 2, 2012 4:20:06 PM
just stick with q6600 . It is a socket 775 processor. U can replace with only another same socket processor. It wont benefit u any to do so as the r all old processors now. U can upgrade ur mother board to latest drr 3 memory type with a g 41 chipset based motherboard which supports 775 socket. g 41 Board memory size is 4 GB for intel board (Note) where as Asus , Gigabyte etc boards support 8 GB.