Core I5 3570K (unlocked Ivy Bridge) for CPU will set you back ~£150 and is at the moment the most cost efficient CPU you can buy. You could splash out another £80 to get the I7 3770K version but at almost half as much again in cost, the gains are only a few FPS (2-10 usually, varies but not much) gains on identical test systems. There is minimal cost difference between Ivy and Sandy Bridge, so you might as well take advantage of the newer Ivy Bridge architecture which offer a bit of extra oomph with a lot less power consumption.
You can get a socket 1155 motherboard using the new Z77 chipset for ~£70 at budget end, which again is worth it and will last you a lot longer than a Z68. If you can spare ~£150 you could get the Asus V8Z77-V Pro board which while a little pricy for a motherboard is fantastic, and will last you a good while with plenty of room to grow.
RAM is cheap chips at the moment. 8gb of DDR3 will cost you about £40. You could get 16gb if you wanted, but as has been said above its pretty much superfluous at the moment, and when you do want to have more its probably going to be cheaper than what it is at the moment! RAM is one of those components that you bulk up on when you need do, due to the market value on them falling constantly.
I agree with Applesnacks last point too, you should definately look at an SSD given their prices at the moment. If you want bulk store go for a cheaper slower spin disk, though the prices on there are quite high since the flooding and show no sign of coming down. SSDs do give great improvement though, worth it for sure!
Edited, Jul 24th 2012 4:25pm by Kordain
To endanger the soul endangers all,
when the soul is endangered it must become a Warrior.