I used a Nikon Coolscan III before, and it worked good, but I think it was a little slow. Even though it used a SCSI interface.
http://www.nikonimaging.com/global/p...an_3/index.htm
But the Coolscan V uses USB 2.0, supports higher resolution and should be faster then the Coolscan III.
http://www.nikonimaging.com/global/p...an_5/index.htm
Either scanner will work with any PPC Mac, no Intel support
http://support.nikontech.com/cgi-bin...i=&p_topview=1
If you want to save some money, maybe buy a used Coolscan III, and a SCSI card for your G5.