I've run Vuescan on G3s, G4s, and G5s.
I'm currently running three scanners off a dual 2.7 G5 using VueScan, including one old Polaroid SCSI scanner. It takes about an hour with my Epson V700 to scan and process a 4x5 at 6200 dpi using Digital ICE, but gets the job done.
I'm a big advocate of VueScan. BTW, every version is available for download on their site-just find the newest that will work with the hardware you have.