It really all comes down to how much you're willing to spend and if later upgrades/customizations are important.
Dell does make excellent gaming PCs (since they just bought Alienware, it's only going to get better). However, they tend to use proprietary parts (motherboard, power supply, case, etc), so later upgrades can be difficult or impossible.
Alienware is one of the top names in the gaming industry, but they also tend to get a little proprietary. They are also a little more expensive.
Usually voted the best in the performance category is Falcon Northwest
http://www.falcon-nw.com
Their PCs are VERY expensive, but worth every penny if you can afford it.