I have no ideas about Linux, but I am running Dom2 on a laptop with SuSE Linux9.2 and a Radeon Mobility 7500. I tried to install different drivers from ATI, but that did not help. Eventually I noticed that the SAX2 config-tool has folder with a checkbox named "enable 3d acceleration"... and now Dom2 runs real fast!
So sorry I this is no big help, but it was all that I was missing...