Well the Source engine is optimized for ATI cards. At least the games from VALVe are. It's just like how Doom 3 runs more efficiently on NVIDIA cards because of OpenGL.
I definately cannot wait for Half-Life 2 for the Xbox 360.
It comes with Half-Life 2: Episode One, Half-Life 2: Episode Two, Portal and Team Fortress 2. All those games for $60. VALVe even optimized Half-Life 2 to use the Xbox 360 multi-core architecture so you'll see more graphical features than you did on the PC, like HDR.