Due to ISA having a much smaller bandwidth than PCI, there won't be any advantage to running DOSBox through an ISA sound card in Linux other than getting an MPU-401 interface to hook your synths up to. Probably your best option is to put the SB16 in the Linux box and use it only as an MPU-401 interface, and then use the on-board audio hardware (which is probably PCI-based) for DOSBox' output.

DOSBox won't make any special use of the hardware on a real SB card as far as I know (for example, I don't think it can use a real OPL chip for Adlib/OPLx music - although I heard someone make a patch for that I guess), so you're only going to open yourself up to potential stuttering problems by using an ISA sound card with modern software.

As for the LAPC-I, do those actually work in Linux?

I like turtles.

Make sure to find a USB MIDI interface that supports Vista 64 if you want to be able to use it with Windows in the future, as many USB MIDI devices I've seen either don't support Vista or don't support 64-bit. I got stuck with a first-generation M-Audio MIDISport Uno that M-Audio won't support any time soon (if ever) because they later released a plug-n-play revision that doesn't need drivers. I hear it works in Linux though since the Linux drivers are written by users.

I got stuck with a first-generation M-Audio MIDISport Uno

Hehe, me too. Switched to Cakewalk/Roland UM-1G which has better driver support (and plug-and-play compatibility as well).

Yeah I went to Fry's to look for an E-MU/Creative one as a replacement but noone could find it on the shelves even though their site says it's available for in-store pickup. I'll probably have to buy one online.