Listen, I've been debating whether to go for i5 750 or i7 920 for a while now, and I've decided to go for core i5. Core i7 is great for your e-peen, you can go around and tell everyone you have one and feel good- but the fact is it will get old sooner than you'd like, and you would start feeling regret at having spent the extra money and not even utilising it's full power.
Are you a profesional video editer? If so, get it, the hyper-threading will work nicely for you.
If you're just a typical "hardcore" gamer, Core i5 will do you absolutely fine. The difference in framerates in games between i5 750 and i7 920 from what I've seen is very close, perhaps 2~ fps. Is it worth spending £100+ just for a 2fps increase? I also saw benchmarks showing the Core i5 with higher FPS results.
As for futureproofing- are you going to buy an i7 now and then spend £400+ on the new core i9 when it comes out next year? That just seems stupid.
X58 is better for SLi / Xfire, but if you're gaming at a resolution of 1600 x 1280 then you won't even NEED to go SLi... you could get a single powerful card and be just fine, which will work just as good on a P55 (Core i5) motherboard.
6GB ram vs 4GB ram- you can just add another 2 sticks of RAM when you eventually need it. 4GB of RAM is fine for now. But you could take it to 8GB if you want, on a P55 motherboard.
Basically, decide if you want to give yourself limits or not. There will always be something more you can spend your money on, "£1000, but if I get this... £1050... well I might aswell make it £1100... oh what the hell £1200 won't hurt..." SET LIMITS, unless you're rich.
Decide on what will be of USE to you and go for nothing more imo...
