I am in a similar situation as you. I have a beige G3 tower and want multiple hard drives for it. I first tried the Sonnet Tempo card only to find that it disables my ethernet and had a "known issue" with DV video. I am looking into the Promax and VST cards that allow for RAID.
To get a 4th drive in my system, I am taking out the floppy drive. I never us it anyway so I don't mind. My system had an internal zip that I put in a cheap $30 SCSI case to free up that bay. If you really needed a floppy, you could always buy an external to hook up to a USB card. I know it sounds strange, but it might be cheaper than buying external SCSI or FireWire drives.