After having the original NVIDIA card die on my DA, I've been running ATI cards exclusively. Am currently running a reflashed Radeon 9700... Works fine.

To actually provide a speedy Leopard experience, I'd consider bumping up to a faster G4 processor. Currently have an OWC Mercury 1.2GHz G4 processor in mine. That's what made the computer seem like it wasn't being bogged down by OS X...

-J