I would say PC world (dont swear!!), I saw an ad the other night on tv for a core2 based system for about £600. BUT you wanted GOOD customer service not sh** service!!! Yes I cant stand PC world:mad:
My advice would be to have a crack at building a system yourself, works out alot cheaper.
You could try scan-
http://scan.co.uk/
they sell ready built systems, also parts for self builds and to top it off, good customer service..