The best PC to play a game on is one you build yourself, that way you know the specs and hardware. Though, in terms of cost effectiveness, it is sometimes cheaper to buy a prebuilt system. If you're not ready to take on that endeavor, I would suggest some of the previously mentioned companies, especially ALIENWARE(
http://www.alienware.com), expensive but top components and a great warranty. Falcon Northwest(
http://www.falcon-nw.com)is another one.
PC Gamer and Maximum PC magazines just had special issues on building your own gaming system. You can consult those for component specifications and recommendations. Also, consult a site called Extreme Tech(
http://www.extremetech.com). They always have good reviews on systems, and their ratings are considered to be very reliable. I'm building a new PC primarily to get ready for STAR WARS GALAXIES in 2003. OH YEAH!!!!
Dell makes good systems, but their tech support has really declined in quality, from what I understand.
Gateway. . .well, they have been known to use refurbished parts.