I would not spend a whole lot on a 3640. Thankfully, they are quite common and cheap with people having gone to '060 and PPC, so for a small sum of money I would definitely upgrade to the '040 card.
Years ago, before I upgraded my A4000 to '060, I traded the '040 card for a '030 (both stock Commodore) and the guy threw in a 4091 to make up the difference.
While I enjoyed the SCSI card, I regretted the trade and I missed that '040 greatly, heh. About 6 months after that I bought a gfx card and '060 for the machine.
