I personally went quite high end, but there are few things you might want to consider
Firstly, google the "digital foundry PC." They made it for £300 and offer up suggestions to make it better
Secondly, don't worry about an i7, the GPU is nearly always the bottleneck, I have a 670 and an i5-3570k. I have never had any need to overclock it to make up for the much better GPU