Does it work when you boot from Live DVD?
I've used several ATI cards and they have all worked if i included in the boot parameters
line xdriver=ati opengl=ati.
(Sometimes you have to include your monitor settings like res=1280x1024 refresh=50-80
or whatever your resolution and refresh rates are.)
Usually if you can get it working from the live dvd and then install, it will work.
If you search this forum there are 3 files that the experts need to see your output from so they can help you if opengl isn't working:
As written in the subject, if XGL and/or AIGLX don't work and you know to have a supported video card, write here and attach the output of:
eselect opengl show
cat /var/log/Xorg.*.log | grep "(EE)"
I am not an expert.
I hope this helps you.