As mentiond Geforce 4 Ti, but that won't work/fit in the Cube. For our little friend, the Mac Geforce 3 is the best OS 9/X card you can get.
As a sidenote, I finally found an AGP-equipped PC to flash over a stack of graphics cards I had sitting here (Radeon 9000, FX5200, 6200, 6800GT); don't know why I bothered as two cards were faulty (artifacts in DOS/BIOS).