The PII X4 945 runs at 3.0GHz while the X4 965 runs at 3.4GHz. The X4 965 should still be fine for playing games, especially if it can be overclocked, but if you are buying a pre-built PC, then the motherboard may not support overclocks. If it's Dell, Acer, HP, Lenovo or any other large brandname, then forget overclocking.
The X4 945 is about 12% slower than the X4 965. Are you save at least that much money?
The 945 is £30 cheaper which will allow me to get more ram (from 2gb to 8gb). I am buying from Arbico so I assume they allow overclocking. As long as the CPU dosn't bottleneck the GPU then I don't care. So will it bottleneck?