The FTDI FT2232 might be worth considering as an alternative to the
Cypress
FX2LP. It's a lot cheaper and smaller, and might be fast enough:
http://www.ftdichip.com/Products/EvaluationKits/FPGA.htm
The Silicon Laboratories CP2102 is also a reasonable candidate for an
RS232 <-> USB interface.
<http://www.silabs.com/tgwWebApp/public/web_content/products/Microcontrollers/Interface/en/interface.htm>
The advantage of the FX2 is that it is more than just an serial
converter; it is a programmable 8051 microcontroller that can execute
algorithms, such as programming the FPGA from a bitstream.