By default, all three systems have their guts stuffed in the Black Ops Assassin chassis, but more "modest" options, such as the Cooler Master HAF 932 are available at a $100+ savings. Every default configuration feature the 2.8GHz Core i7 930, 6GB of DDR3 1600MHz RAM, a 500GB HDD, a 750W PSU, an Asetek liquid cooling system, free CPU overclocking to 3.9GHz, and Windows 7 Home Premium 64-bit.
Despite the significant price leaps, the GPU and motherboard (all X58-based) are the only two components that seem change between price points. The Performance PC has an Nvidia GeForce GTX 470, the Enthusiast rig has an ATI Radeon HD 5870, while the Extreme machine has a GeForce GTX 480. All systems ship with a three-year parts and labor warranty and should arrive by May 25 if ordered today.
If you're more of a DIY kind of person looking for pointers, swing by our desktop buying guide which is always kept up to date.