The "MultiBIOS" is a multi-region bios. when the multi bios is installed you can change the region of your NAOMI by changing the dipswitches.So to get multiple games working, I need the multi bios? Will that also mean that if I want to switch from a USA only rom to a Japan only rom that I'll still need to physically change dip switches on the Naomi?
All official BIOSes are 1 region only and the dipswitches have no effect. AFAIK the Japanese region has 100% compatibility with all games, but the USA and other regions are useful in order to play the games in English... unfortunately there are some games that will ONLY run with a NAOMI set to the Japanese Region such as Street Fighter Zero 3 Upper.
There is a games list I linked to at the beginning of the guide. that has a spreadsheet with which regions are supported by which net-bootable games, as well as compatibility info regarding I/O boards among other things.