well, what do you want to do with the system?
for now the system in your sig should be handeling basically anything you throw at it, if you just wanna make crysis smoother then just get an extra 2gb of ram and one of the new cards, probably the 4870X2
Apprently Nehlem(core i7) will not change game performances much
with £2000 it should be pretty easy to build almost any system you want.
If you really want i7 then (this is me guessing, and they are very vague figures):
~£250 for new motherboard (X58)
~£400 for the CPU (the middle of the new range)
~£300+ for 8gb memory
~£350 for 4870X2
~£100+ (your current PSU will be fine for '1' card, but if you ever decide to go x-fire I think more will be needed)
thats ~£1400 (allowing £200 for price fluctuations since I havent seen any UK prices anounced for the new hardware) that gives you £400spare, you could get 2 SSD (around 64gb) for this and put them in raid 0
at that kind of budget it would be nice to know what you want the system for mainly so that it can be tailored slightly more, as I say, for gaming I see no real need to spend that much
Personally I have found a system built for 1/2 that price can last for ~4years