Bandwidth requirements are dependent on the compression you will use on the IP circuit. But frankly I am yet to see any ASP working efficiently on less than 22-24k per seat.
Theoreticaly they should work on either of 6K, 12k, 16k, or 32 k compressions. A safe bet will be around 30kbps with data and screen pops.
Regards
Saeed Qadri