Radeon vs GeForce.. Which is better for gaming?

Well on Monday i am wanting to spend about £100 on a new GPU. Now i've always used GeForce but since this upgrade will bring my GPU up to spec with the rest of my rig and be a pretty good gaming system i want to make the right decision but i am really unsure as to which brand to select. I don't want to get Radeon if it is going to have compatability issues.

I don't even know which model to get, i've been through websites like ComputerOrbit and PC World and they're pretty good prices but, as you can understand, i will be paying £100 for it so i'd like to get something for that price but that's also moderately good.

My rig, so far is:
GA-77OT-D3L motherboard
650 WATT PSU
2Gig DDR3 RAM
3.01 Ghz AMD x64 Quad Core

So something that would go nicely with that would be good, i know i can play Crysis on my current rig but my current graphics card is really letting me down, i can play most modern games on it with pretty good settings but yeh my GPU is letting me down

Any suggestions?

Thanks