www.ebuyer.com
www.dabs.co.uk
www.planetmicro.co.uk
www.microdirect.co.uk
ebuyer and dabs are the best 2 in my opinion
also, i think go with an AMD cpu, as they will give better performance in the long run. also a new motherboard may need to be bought according to the cpu you do buy (unless you have it already). 1Gb of ram is good enough at the moment