Yeah, around £20 for the CPU (cheapest hex-core vs i5 2500K, the i7 is not required to be honest for 99% of uses), RAM is obviously the same, but the big killer is the motherboard. Cheapest P67 SB motherboard is £99, while the cheapest AMD 8xx motherboard is £50 - and that will allow fair overclocking too.
However, the i5 is quite a lot better. It depends whether £70 extra is justifiable (also, it depends what AMD motherboard you'd actually pick, as I doubt you'll get the cheapest board available for overclocking), or whether the £70 could be put towards something more relevant to your uses (if you're gaming, it's pretty much the difference between a 6870 and a 6970 for instance, or a GTX560Ti and a GTX570 - in which case, the GPU wins out).