I think the common consensus is that a Mac flashed nVidia GeForce 6800GT is probably the fastest, easily-available card you can run. (see https://forums.macrumors.com/threads/176957/).
That said, I'm running an ATI Radeon 9600 Pro 256MB in my MDD G4 and it's a really nice card - I run it about 20% overclocked and even then it's completely stable and runs really cool. I paid about £35 for it delivered.
GeForce 6800GT is one of the cards that doesn't play nice in a MDD. You could use a flashed ATI X800/850 or an ATI Radeon 9700 Pro. The 9700 is a rather rare expensive card.