cossy :
i am willing to spend £100 on a MB and about £180 for CPU. Not sure what a microcentrer is.
Points:
1) If you're going to replace the motherboard, you might as well go Intel and get the i5-3570K (or similar) CPU and a Z77 1155 motherboard.
2) When you replace the motherboard you will be required to purchase a new copy of Windows since it's very likely you have an "OEM" copy. That's more money (if so, get Windows 8 64-bit and Start8 from Stardock. Windows 8 is a big improvement such as Secure Boot to prevent boot-time malware hacking like key-logging.)
3) The performance increase will vary significantly depending on the game. Many games are more GPU-dependent so keep that in mind as you may not see much difference in many games. Plus, if you can achieve 60FPS at near full quality anyway don't upgrade. I often tell people to just tweak a little rather than spending a lot of money.
4) If you do go with Intel, you'll need a new cooler (don't use Stock) but that's not a huge part of the upgrade price.
5) *Unless you can achieve a significant improvement by upgrading ONLY the CPU, my advice is WAIT and build a completely new system in a couple years. I find it difficult to recommend spending a lot of money when you currently have a fairly nice rig (not the best, but still quite good relatively speaking). Do a few small game quality tweaks and any game should play quite nicely.
Cheers.