I am running a 700Mhz G4 eMac, with a Geforce2MX (32MB) and 640 MB of system RAM. Performance may, in some cases, be very slow (single digit fps) but usually is reasonable (10-40fps) for most areas. IMO it is important to have a good chunk of RAM first and foremost, and after that a decent graphics card is helpful.
This is all based on system 10.2.8, I believe newer graphics drivers in 10.3.5 may yeild better performance.