Either a Quad, Tri, or Dual core processor with a large L2 cache, a high RPM hard drive, 4 GB of RAM (if you're using a 32-bit OS, preferably XP), and an nVidia 8-series graphics card, preferably dual 8800s if you can afford it.
Why do I say 8-series? Because nVidia bought Ageia, the company that makes physics cards, and nVidia will bring physics capabilities to all existing 8-series cards through a simple driver update.
You'll need a quad 6600 chip link this:
http://www.amazon.com/Intel-Q6600-Q...1?ie=UTF8&s=electronics&qid=1211615669&sr=8-1
With about 4GB ram
Like 2 500GB HD
8800GTg graphics card:
http://www.amazon.com/EVGA-e-GeForc...1?ie=UTF8&s=electronics&qid=1211615744&sr=1-1
If money isn't an issue:
http://www.alienware.com/product_de...?SysCode=PC-AREA51-ALX-R8&SubCode=SKU-DEFAULT
Just FYI, Dell owns Alienware, and HP owns VooDoo.
Dell is replacing their gaming XPS line with Alienware products (soon). Their XPS line-up will become the more "aesthetic" line of computers.