https://bugs.freedesktop.org/show_bug.cgi?id=28995
--- Comment #16 from Dave Airlie airlied@freedesktop.org 2010-08-07 15:56:47 PDT --- http://git.kernel.org/?p=linux/kernel/git/airlied/drm-2.6.git;a=commit;h=e37...
might be helpful or you need to increase AGP aperture in BIOS if you can.
also it might be worth trying to boot with radeon.agpmode=-1 as a test.