Every game lists on the outside of the box what the Minimum and Recommended system specs for the game are. You can usually find that information on the internet, too. Just checkout the system requirements for the games you intend to play, and design a machine that will meet or exceed the Recommended requirements. Ignore the Minimum requirements because they only get you to the level of Barely Able To Function.
Then, order your stuff from NewEgg.com. They have the best prices and the best service and they have customer feedback like on eBay, so you can see what others are buying and what their satisfaction rate is. (I don't work for NewEgg, by the way. I'm a happy customer.)
In a name brand desktop, how little can I spend and still get good gaming performance. I've read about quad cores and the new 8800 graphics cards and physics accelerators. But lets take it down to decent performance not the "gamephile" astronomical prices, for little bumps in speed or better looking leaves! Let's say a Dell xps410 or lower and $1K or lower. Any ideas?