In order of lovelyness:
Intel i7 - quad core - very expensive (£250+ for chip alone), plus memory more expensive as its triple channel (requiring 3x memory chips). Fast but expensive.
Intel i5 - quad core - mid range cost (£150+ for processor), nice performance/cost.
AMD X6 - hex core (6) - about £170+, higher clock speeds but not really comparable to Intel i7, but better than lower clocked i5.
AMD X4 - quad core - £100+, very high clock speeds, fantastic performance compared to cost, similar to lower end i5. AMD 965 black edition (£120) would be my recommendation.
Not convinced about need for 6 cores, as this is overkill for most homes, unless you are a major games freak or running linux, other virtual PC stuff.
Intel i7 is the argument settler, but again, this wins on numbers, but in real world, is it really necessary? Does it matter that Word takes .7 secs to load compared to .73 secs?
I have only just upgraded my PC, and have always treated myself to the good stuff, but found that the quad core AMD, nice motherboard, 4GB memory all came in for £280. Then went out and upgraded the PSU (as these new MB's require ATX 2.3 spec with a special 4+4 plug on the motherboard, although converter from lesser spec is available).
And then the most important upgrade, a 60GB solid state disk (OCZ Vertex 2E for £100), my god, it's quick. 20 secs to load Win7. Save a little on the CPU and splurge on the disk.
