horsepower!! The 1600R series is for a small office.. 20-50 people assuming a full T is used.. I have run 2,000 users through a 2621 with a full T1 for web access. Your milage may vary depending on a varity of things. If you do ANY bridging, run extensive access lists and so on, you will limit the number of users you can support due to CPU loading. If you run NAT, keep in mind this can add substanially to the CPU load.
Myself, I would offer a standard package of the 2611 or 2621.. that way you know you are pretty well covered.
Mike S
"Diplomacy; the art of saying 'nice doggie' till you can find a rock" Wynn Catlin