A 2851 can't guarantee to provide 80 Mbps (duplex).

How powerful a router you'll need would depend on actual traffic bandwidth utilization, typical packet sizes, configuration of services, and how many routes your BGP will be processing.

For 80 Mbps (duplex), you might need something like a 3845 or 7200 with a G1 engine.

Depending on your feature needs, a L3 switch might also be considered.

I've attached a Cisco reference sheet which provides the preformance of many of their different routers.