I think you'll find the HD 4850 is best for price-performance at the moment.
Consider this with a HD 4850, an OS, 22" monitor & speakers included in the price of £749 - Chillblast Fusion Hornet click here. Take the £15 PSU upgrade & the overclocked Q6600 for -£5. Total = £759
Though higher clocked dual-core CPUs are generally better for gaming (& would normally recommend an E8500 over a standard Q6600), in this instance I'd be inclined towards the latter's overclocked version in the above PC. G
P.S. cube 247 forum search click here.