I fixed the weird DIMM problem on the free computer. They were hard to stuff into the slots, and I thought that was causing them to bend and not make good contact. So, I decided to test the theory. I took my handy-dandy Dremel tool, mounted a cylindrical mill bit on it, and ground the edges of the DIMMs down a bit until they slid easily into the sockets. Result: System came up with 512 MB, no sweat.
There are some problems for which a Dremel is the right answer. That number is larger than you might guess at first.
Unfortunately, it didn't fix the SCSI problem. I've ordered another card off eBay; we'll see what that does next weekend, most likely.