@Joe Glaser
As said above, an easy workaround is to disable the r600 shader optimizer. Just
launch Steam like this:
$ R600_DEBUG=nosb steam

But performance will be not good, and you already have a weak GPU. If it's too
slow, then you can try to patch and build Mesa. You can follow this tutorial:
http://forums.fedora-fr.org/viewtopic.php?pid=532589
If you don't read french, you can translate the page with Google Chrome. The
translation is still... understandable.