We've tested several USB->Serial (rs232) converters. Many of them have poor driver support and are not designed for high speed continuous use. Many do not support hardware handshaking either. We have found that the Belkin F5U109 (a very inexpensive model) works perfectly and also supports high baud rates and hardware flow control. The com port to use with these is usually com5 or higher (see iceconfig).
Also, user reported that you can share a single usb serial convertor with 2 PC's connected to a Unix host on one serial line (of course you can't use them BOTH at the same time). A neat trick so I thought I'd pass along the comment....
The belkin F5U109 (usb - serial converter) together with the F1U201 Peripheral 2 port switch (2 to 1 usb device) worked really well with Ice Ten to switch between 2 computers to 1 serial port.