Yeah the days of going to computer fairs building up systems to sell are long gone, got me through uni though
You only needto look around, 5-7 years ago even the smallest village had a little PC shop but you don't see many now
For £400 you can spec a system that will be everything you want Neil. I have used yoyotech recently mainly because they are in London so any problems I could get there but they do mail order. You spec a case + processor and pretty much everything else is customisable....
1,000s of other similar sites and shops though, if you wanna chat PM me, I'm sure Chris can give lots of good advice too