I'm currently using the internal 16550 based serial port on my ASUS
P5A-B motherboard to drive my ISDN terminal adapter (3Com ImpactIQ) at
115 kbps on a single (64K) ISDN channel.  Since prices have come down,
I'd like to use both channels (128K).  The ImpactIQ will support a 230
kbps speed on its serial interface.  The ASUS interface doesn't support
that rate.  I am running NetBSD 1.5.1-ALPHA.  From my research, I now
have four options:

1) Do nothing and run at 115 kbps.
2) Get a USB to serial translator.  I found a unit at a local vendor for
22USD that supports speeds up to 1 Mbps.
3) Get a two port PCI serial expansion card using a 16650 chip that
supports bauds up to 460 kbps.  About 50USB at a local vendor.
4) Get a one port PCI serial expansion card using a 16950 chip that
supports bauds up to 920 kbps.  About 50USD at a local vendor.

Reviewing the hardware compatibility on the NetBSD site, I didn't see
any support for a 16650 or 16950 chip.  There did seem to be support for
a USD to serial interface.  Does anyone have experience to know which of
options 2 through 4 will work?  Thanks in advance.