I highly recommend IBuyPower. I’ve bought my last three computers from them, and they’ve all been excellent, and considerably cheaper then most other options, which is always great. Their site is a little flashy, which is annoying, but the computers are very good, and I’ve never had an issue with tech support.
For a reference, I built my last computer from them for ~$1200 last year, a with a 2.6GHz quad core AMD, 2 Gb ram (a little low now, but still), a 21” LCD, A high-end radeon, (HD3800, i think? Not on that computer now, i apologize), 120Gb HD, and XP (in my opinion, it was a good sign they were still shipping with XP at no extra cost, when many weren’t), quite a few extra deals (a few games, like AOE3, etc), an excellent full aluminum case, and other random stuff.
It’s an excellent computer, can play Fallout 3 on full settings no problem, and it was only 1200. So, yeah, I like them.
@johnpowell Buying parts and building it yourself or with someone else’s help is always the cheapest option, and typically the best in terms of performance, but it can be difficult to find someone if you don’t know someone already, and can be a real issue if you need tech support and didn’t build it yourself.