The Radeon 9000 Pro is going to be the most commonly available G4-ADC 9/X accelerated card.
There's also the Geforce4 Ti4600 which has G4-ADC, very quick acceleration in 9, but only emulates Quartz Extreme in OSX (so isn't a great choice there despite strong numbers.) It's also noted to have a noisy VGA output; DVI(ADC) is fine though.
The MDD series will also have had the Radeon 9700 Pro as an option which is significantly faster, retains G4-ADC, but only accelerates in OS X.
Edit: I've also heard from multiple IT offices that running displays off the ADC port contributes to PSU failures, especially when machines are left in sleep mode overnight. I run mine off of ADC too (too streamlined not to,) but I'd switch to an external brick if I were going to run one of my G4s heavily.