The Modem slot though, has 5 bit addressing with an 8 bit bi-directional data bus, including a Read/Write line, plus an IRQ line. It also has a Chip Select line, so that tells me its bus has to be mapped through one of the VIAs, or System I/O controller.

Should be sufficient for a basic NIC, but if i wanted to integrate, say, an SD card reader or something other, it might slow things down, unless i hooked into the RAM address slots, which give me direct processor access.