Serial ports are stuck basically at 115200, but with ISDN, you can get 128000. Do the math, a basic serial port (with a 16550 UART) cannot handle the extra data coming down the pipe. This isn't a major problem, but if you want to use all the bandwidth, you will need a higher speed serial port to handle this (16650 UART).
If you get an internal ISDN modem, this is all solved, so its all good. With Win98, the default DUN (dialup networking) component supports the multilink, so you'll be set!
Did you know (at least a few years ago) that in California and New York that ISDN wasn't 128k? It was 112k (56kx2)! Its not a well known thing, but it was true. I had ISDN at 112k, it was fast, but DSL is 10x better now.
vash