Not sure what driver version you are using. The latest releases (1.0-43xx) support FX based cards. It really doesn't matter what card you select in SAX2, it is a Riva TNT or a GeforceFX 5800 .... all use the same driver. (make sure it is the latest release)
Oops didn't see you talked about a GO version. Not sure if those are supported yet atleast the non-GO versions work...