I've heard plenty of anecdotal evidence of battery-backed RAM living longer than it's supposed to. I don't remember the design of the card case so I don't know about replacing the battery. However, it's not required that you use an SNK-branded card; the memory card interface is supposed to conform to a PCMCIA/JEIDA standard;
Digi-Key has a few cards that seem to conform to a more recent (4.x, rather than the 3.x referred to by most pages I saw on the subject) revision of this standard. The cards can be found in their online catalog on page 290, but they're a bit expensive.
[ June 17, 2001: Message edited by: Ex-Cyber ]