I took out the top SO-DIMM and left the bottom one in. The system wouldn't boot OS 9.2. I put the other SO-DIMM in the bottom slot and it booted 9.2 and X fine.

In OS X I ran Disk Utility to "Verify Disk Permissions" with the good 512 MB DIMM in place. It ran fine. I ran "Repair Disk Permissions" and it ran fine.

One of the SO-DIMMs is flakey. :-( They're Kingwin parts (discussed in the OP). I'll probably order a couple of [link|http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=4AEBAB1EA5CA7304|these] in a day or few.

Cheers,
Scott.
(Lesson #2: If you suspect bad RAM, spend some time checking whether it's good or not rather than spinning your wheels with software checks.)