While it's impossible to state with 100% certainty what load will be generated and as such whether or not you'll run into the limitations of a given box when talking about traffic, due to the completely variable nature of one deployment vs. the next, that doesn't sound like much of a load for a 1600. 15Mbit/s isn't anything to worry about, certainly. When you say 4-5k sessions, do you mean concurrent connections, requests per second, something else? What value are you measuring here.
We can't really state sizing info with 100% certainty without doing actual tests against real traffic on a real box, but that doesn't sound like anything that I'd worry about.
Colin
Cirrus