Scan.co.uk is cheaper on almost everything 
+1 I use Scan for my machines.
Make sure you get paired ram like Corsair, with fast operating speed
4gb of crap ram is a lot worse than 2gb of paired, also make sure you have at least four slots on your motherboard for upgrades as 4gb sticks are far too expensive.
My last machine went on for another 2 years becuase when I speced I made provision for running another GPU in SLI config

I currently run a 3.2 quad core AMD Black edition running overclocked, ATI something something GPU with 1gb of onboard memory + 4gb of paired corsair.
I can play all my games like DOW and COH on full chat with no lag or frame rate issues, I am lucky also to have 20 meg internet

The more you spend the longer your machine will play the newest games, but stay away from Alienware or other overpriced systems, you can buy the same spec from scan or O/C for a fraction of the price, who needs an iluminated case with some sh!t alien face on it.......