Well, I got a 1GB DDR DIMM (Patriot PSD1G333), and two 4" putty knives, and installed it. I'm glad I bought two knives - it was a pain to get into the case. Anyways, the upgrade seems to be working.
Those putty knives are sharper than I thought - it took me a couple of seconds before I clued into where all the red smears on the Mac Mini case were coming from. I guess I should stick to software. :-)