Over time I've come across a lot of very nice PC builders.
Also, if you walk into Fry's or Micro Center, they will have a custom built, high-end computer on display. Usually >$6K and totally tricked out.
However, there are reasonable, high-performance PCs to be found if you know where to look.
And I don't put Alienware in this category, ever since Dell bought them out, they have been subject to some of the big corporate cost- and quality-cutting that large corporations inflict on their minions in the name of efficiency.
Here is my list: (not even in alphabetical format)
Maingear: http://www.maingear.com/
Digital Storm: http://www.digitalstormonline.com/
Origin PC: http://www.originpc.com/
Falcon Northwest: http://www.falcon-nw.com/
Puget Systems: http://www.pugetsystems.com/
CyberPowerPC: http://www.cyberpowerpc.com/
Velocity Micro: http://www.velocitymicro.com/
I agree that it's probably fair to take Alienware out of the list. That said, if you want a powerhouse some of their stock models are worth exploring. Their Customer Service has gone to shit though.