which, if you are lucky, will be bundled with the Sonnet upgrade you find on ebay. Otherwise the onboard video or a Nubus video card should pull adequate duty.
Further information can be found at www.kan.org/6100 under the appropriate headings. Although the site is specific to the 6100, which can have -either- a PDS card -or- a Nubus card, but not both, the information there should be helpful as the basic machine architecture is the same.
There were other makers of G3 upgrades for Nubus Macs; I believe Sonnet were the only ones with the PDS pass-through cable. There were also upgrades that would only fit in the 7100 or 8100 (or 9150), and not in the pizza-box 6100.
http://68kmla.org/wiki/SCSI_harddisk_replacement_options
http://lowendmac.com/ppc/workgroup-server-9150.html