Hi,
The FX5200 that are flashed on Fleabay are not in the same league as an ATI 9800-256. Don't be fooled by the 256MB, it is the Core Frequencies that are important (GPU speed/Memory), the ATI is MUCH faster. The ATI 9800-256MB Mac/PC Edition is the fastest retail video card for a G4.
Another little detail is that flashed FX5200's don't push large LCD monitors very well. Anything over 19" is 90% not going to work. An ATI 9800 (even flashed ones are good) will push any LCD up to 24" easily.
I have all of them, flashed and Mac Editions. For a Mac tower system get an ATI 9800, either flashed or Mac Pro Edition - 128MB or 256MB. You wouldn't be sorry.
Regards,
Dave