Xbox has alot of of it's visuals turned down vs PC.
Most games look like PC games set at 640x480 low qual. Xbox 2 is reported to include full sceen AA for HDTV res games. Not sure if the system is going to beat some of the 6800 ultra machines out there though (speculation based on r520 forcasted fill rate only).
It's definatly going to be one heck of an improvement over Xbox 2. Should be avail for christmas 2005, which by then we'll be well into SLI land.