Hiya,
I would give a massive hearty recommendation to any system built by Best Price Computers (Poweroid) -
http://www.bestpricecomputers.ltd.uk
Their web site might not be the best looking around, and they aren't the cheapest, but the build quality is simply superb. I bought my main work PC from them two years ago through TGC and it has been on virtually 24-hours a day, every day since with no issues at all. All I changed were the graphics cards when NVIDIA shipped me a new ones.
Very recently I also replaced the motherboard, but it wasn't because it had died on me, simply because I needed the features of newer boards. I would have no hesitation at all in buying a new PC from them again and plan to do so once Longhorn is the "defacto" standard in a year or two.
I know their company name is cheesy, but honestly their machines rock.
If you want to build it yourself instead, then I wouldn't shop anywhere other than
Overclockers UK. Great service, prices and speedy delivery. I hate looking at their web site though, because every time I go on there I end up spending money
Cheers,
Rich
"I am not young enough to know everything."
- Oscar Wilde