You can either stretch your budget to get one or the other of the Antec 902 or a 5850 (minimum for a gaming system, which would only require a 650W PSU) or both, or we can explore basing this system on the i5-750. Where is it much more likely that you will get everything in.
Generally I will suggest ATI cards as they usually offer better performance over the nearest Nvidia price rival, and use less power.
The best way to get a very good gaming card into this budget is to change to an i5-750 system, you would spend less on CPU, similiar amount for a very good P55 motherboard that can crossfire, less on RAM. Those changes in the system I suggested would bring the total down to around £850, which is still a bit short to get a £230 5850, but you would only be £50 over budget.
The only real reason to stay with an i7 920/930 is if you want this to be the basis of your system for quite awhile, and don't mind cutting back on the GPU for now.