Core i5 2500k - One of the best processors ever released. Up there with the Q6600 and the Athlon64s
If you get one with a Z68 motherboard, when the time comes, you can drop a 22nm Ivy Bridge processor right in there, no BIOs update, no messing. Just drag and drop, literally
And if you have a GTS250 upgrade it to something like a 6850. They're around £110. Or a GTX 460 1gb, which are a similar price. MASSIVE perfomance boost.