Some Notes on the FTDI part

in general this part is good replacement when you are trying to emulate the RS 232 port unter W32. There is no support under DOS.

One thing which I recognized when testing that device:
It will be reletively slow if you are transmitting relative short messages to the device and you need a response.
This can be a problem for some implementations.
Controlling my serial modem with that chip showed good performance
But trying to control my device programmer (ALL-11V) slowed down the speed to efectively 2400-4800 Baud (at 115k)

Personally I am using TIs TUSB3200 which has audio support butgeneral enough to do other transfertypes too.
Another serial usb chip would be th PDIUSB11 from Philips with I2C interface.

Thomas