Okay, done a bit of looking around and it seems Parallels 3.0 only supports up to DirectX 8.1c, so that could be an explanation. As said, Quake 3 engine games run perfectly at all the highest setting, so I'll wait for a while before attempting anything else. Still, seems a bit misleading to me for the developers to post images and videos of Quake 4 running under Parallels, but I'm seeing a lot of those kind of complaints.