My son had a gaming laptop built to his spec by alieneware. At the time he loved it but stilll 'outgrew" itand then went on to build his own computer. Sounded difficult to me but he was 13 at the time and it didn't take long.
I second the chillblast recommendation - their desktops are very reasonable, components are customisable so you can adjust to suit your needs (and they then check that what you've chosen will actually work...), but in particular their customer service is excellent. My pc from them had a couple of issues, and while it was in warranty they picked it up, replaced parts, and returned it on a schedule designed to fit around my holiday for my convenience! Even after the warranty finished they provided ongoing support for a necessary upgrade. I'll definitely get my next pc with them too.