For "serious" gaming (hard core or just HD) there are two options, XBox 360 and PS3.
XBox was released first and for a few years had a much better collection of games. It also has Halo, which is a cool game. The on-line gaming experience is better on XBOX but you have to pay for it. Kinetic was just released (image based game control).
PS3 is technically a superior platform, it's built in Blu Ray player allows game makers to build massive games, and you can play DVD and Bluray movies (including 3D if/when that becomes relevant). The biggest advantage the PS3 has is that every unit has a HDD drive, what that means is that game developers can depend on it as opposed to having to design their game performance for a system (XBOX) that may or may not have a HDD. Another nice advantage is the bluetooth controllers and remots and now the MOVE system (optical version of the WII remote). Sony has many exclusive games such as Metal Gear Solid, Ratchet and Clank, Grand Tourismo....
As you can tell I'm more of a PS3 guy, it's rock solid (google red ring of death for xbox), is the best movie player you'll ever own and the best selling serious (ie not Wii) game platform.... I own 2.
OTB