www.scan.co.uk is also good, just don't look in their dream PC section (currently their flagship model, The JellyFish, sells for just over £7000)
Being water-cooled, upgrading can be a pain, upgraded 2x 8800GTS to a single 470GTX from EVGA with mounted waterblock, cost me £475 :s
I'd say a good stabil PSU would be my starting point, as then with every upgrade you make, you won't have to worry about blowing your current PSU.
I got an Antec Quattro 850w and it has served me very well.
Powers two water pumps, eight 120mm fans, one 470GTX, overclocked CPU and RAM, six hard drives (5TB total)