The P35 P5K I really recommend, it has brilliant OC'ing capabilities (better than any Nvidia chipset) and costs around $110-120. Supports Crossfire and some models also support DDR2/3.
P35 or X38 chipset is what to look for, I can't see the sense in 650i, 680i or 780i boards if you're not going to use SLi. (which seems to be turning into a bad joke imo)
Asus P5K, Abit IP35 Pro or similar, not sure on pricing in the UK.
The P5k is the cheapest P35 mobo in the UK (about £65) apart from the MSI iP35 but that doesn't really have adequate chipset cooling for OC'ing and it's only about £5 cheaper anyway, so the P5K is the best way to go.