Very interesting - I thought that PAC was only the name of the MSX-MUSIC enabled cartridges. I thus limited my searches to "save RAM" which doesn't bring up much - I could have looked for PACs all along! They aren't much use to me since I play the shooters straight through, but it's nice to know. I recall recently seeing a Konami cartridge (I believe), with I think a gold Pentarou or Konami Man on the cover that I thought was a save cartridge, although save RAM cartridges seem rarer than I had thought. Of course, if the MSX-MUSIC enabled PAC is the most common by far, it's probably still best to get that one for cost reasons.
There is one last question I have, then: What difference is there between the Sony F1-XDJ and the XV? When I bought mine, I had the option of getting the XDJ or the XV. I'm happy with mine, but I've been troubled ever since by the seemingly identical features between the two machines. As best I can tell, they both have the same MSX2+ features (the FM chip, one disk drive), the ren-sha and CPU speed sliders, the kanji ROMs, and MSX-JE. I've always been puzzled how Sony could have differentiated the two in the market because they don't seem to be different.