I have no idea why you rushed into buying an old GPU from an auction site. £26 for an old 8800 seems a lot of money considering what some newer GPU's are going for. Many 8800GT's are £14 buy it now. £30 could have probably got you an HD4870 or the like second hand.
If you have updated that motherboards Bios to F12 you can add the Q9550 or such, but you should consider the costs of any CPU, £30 for a Q6600 up to to £80+ for a Q9550. £100 would get you an up to date processor, AMD or Intel. I would guess most integrated graphics on todays CPU's would be more capable than the old 8800GT in many areas.
You are also lacking in memory with only 2gb, so I am guessing Windows XP?
Can you list your actual components? Are they IDE or Sata 1 or Sata 2?
Case?
Motherboard.
CPU.
Cooler?
Memory.
Graphics card.
DVD drive?
Hard drive?
Operating system? 32bit or 64 bit?
Power supply and it's wattage and connections?
What games are you hoping to play?
If the above is seriously lacking then all you indeed have as an option is to buy an old Q6600 and overclock the hell out of it. But what you can do will be down to the CPU cooler and bios options you have.
With only 2GB of memory and such, I think the Q9550 may be taking that old set up too far without a better motherboard, though if you do not need 4 cores of a Q6600 you may find the 2 cores of an E8500 get you further due to higher overclock per core.
I never bothered with the Q6600 as back then many old games never needed it, and the E8500 did better in games for many. I also have a Q9550, but still found the E8500 to hold it's head against that.
Don't get me wrong, I still like and use three LGA775 systems, dual and quad core, but I am sure newer hardware offers more performance per pound against much of LGA775 current second hand prices.
Maybe dropping all the above for something along the lines of what is below would serve you better? With current performance per pound, I am sure you can understand why some ponder spending any amount on old hardware as a main machine.
YOUR BASKET
1 x
AMD A6-5400K Black Edition 3.60GHz (Socket FM2) APU Trinity Dual Core Processor (AD540KOKHJBOX) £47.99
1 x
ASRock FM2A55M-DGS AMD A55 Chipset (Socket FM2) DDR3 Micro ATX Motherboard £41.99
1 x
TeamGroup Elite 4GB (2x2GB) DDR3 PC3-12800C11 1600MHz Dual Channel Kit (TED34GM1600HC11DC01) £29.99
Total : £129.86 (includes shipping : £8.25).
