What's the best gaming computer?

A custom-built computer will most likely work out cheaper for you, and you can be sure of the components in it because you chose them. A few notes to ponder:

Processor: With dual core processors, they are almost essentially two processors in one. So although the clock speed is 2.4GHz, there’s two dies/cores working at that speed each, so theoretically you double that and the single-core (normal type of processor) equivalent clock speed is 4.8GHz. Reality is that its not quite 4.8GHz, but its not far off it - would be no less than 4.6GHz. And if you’re going with an Intel dual core processor, only get a Core 2 Duo, and not just a Core Duo - Core 2 Duo’s are faster, more efficient, and 64-bit compatible whereas Core Duo’s aren’t. You are going to want to have a 64-bit compatible computer - games will take huge advantage of this in the future, many already do. A lot of gamers like AMD processors instead of Intel - any AMD dual-core processor will be 64-bit anyway.

Ram: Depending on what Operating System you intend on running, 2Gb is probably a minimum for a gaming computer. Make sure that the maximum Ram limit of your computer is no less than 8Gb, preferably 16Gb.

Graphics Card: If you’re going to use Windows Vista, make sure you get a good one that has compatible drivers, not all graphics cards do at the moment, even the newest ones.

Hard Drives: Make sure they’re SATA, and SATA II at that. Don’t bother with Nand Flash drives yet, they are really expensive and don’t offer much room at the moment. With SATA II drives your computer will be even faster, especially if its 64-bit throughout.

And stay away from Dell if you can - desktops aren’t so bad as the laptops, but just remember, they’re cheap for a reason.