Interestingly, the ATI chip in the iMac can run with the open source ATI drivers (which are in use by default) and has 3D acceleration working. Just try to enable the desktop effects in Ubuntu 7.04. It should work. It worked great on my 600MHz G3 iBook. Of course, the CPU is kinda slow so the effects aren't spectacular, but they work as well or better than on a a faster PC with an nVidia card and the nVidia supplied drivers.