I'm also in the same position, and have almost narrowed it down to Mesh. But do read some dodgy after sales support complaints on the forums, BUT their kit is top notch using branded components and very good well known motherboards ....
Plus I like their NEC TFT's which have ace write ups, probably the best around ........
Mesh prices are very competetive, their online change spec system for ordering is very good plus with a 3 year warranty I don't think you can go far wrong.
My only gripe is that their machines don't have USB's on the front of the cases and XP come's in the form of a recovery disk.
Also consider -
www.novatech.co.uk or look at
www.dabs.co.uk or
www.jungle.com .......
good luck !