The performance reference that Collin provided, goes far in helping to "size" a router. If you look at the Fast/CEF Switching Mbps column, that will give you an idea of the bandwidth the router can forward. Keep in mind, though, to allow for duplex traffic, and to allow sufficient reserve extra capacity. For the latter, you might want to try to insure the expected CPU won't average more than half to two thirds.
Also keep in mind actual expected load and impact of other processes. I.e. you don't always needs to size for maximum link bandwidth, although doing so, guarantees performance if the link goes to 100%. In your case, processing Internet BGP tables and/or VPNs (if terminated on the router), needs careful consideration (NB: partial Internet BGP do help to reduce CPU BGP scanner load).
For example, the 3845 notes 256 Mbps. Halving that for duplex, and again 50% for reserve capacity, it would probably "safely" support 64 Mbps (duplex). I.e. 3845 could be slightly insufficient to support 100 Mbps (and probably why Cisco recommends it for up to T3s [45 Mbps].)
The G1 or G2 variants of the 7200 (or 7301) would be better positioned for 100 Mbps. (If you don't need the slots, don't overlook the 7201/7301.)
For Gig, you'll probably want to consider the ASR or perhaps a 7304 with NSE-150.
Depending on your feature needs, you might want to investigate the Metro Etherswitches (NB: VPN and memory for BGP might be issues).
