A Maccentral article discloses that Apple's posting info of the ATI Radeon was a mistake:
"For ultimate graphics performance, choose the ATI Radeon graphics card," read the info on the Apple Store Web site.
No option to choose the ATI Radeon card was available to users, however -- the RADEON information existed only in text generated by clicking on a "Learn More" link included as part of the "Graphics Support" option on the Power Mac G4 and Power Mac G4 Cube's custom ordering pages.