Another detail revealed by The Inquirer is that the final clockspeed of the Xbox 360 GPU will be 500MHz. Developers have already received Xbox 360 developer kits with working R500 graphics cards and they confirm they are running at 500MHz. The Inq believes that Microsoft decided to go for 500MHz to get excellent yields. It would be able to clock it at higher speeds but this would render it unable to prepare enough quantity for the Xbox 360 launch.
This means that if anyone finds a way you will be able to overclock the graphics card of your Xbox 360 to improve performance