Hi,
1. IPSEC
2. NAT
3.BGP
In these processes IPSEC and BGP are CPU intensive processes. Most likely those are processed by CPU. But ultimately CPU usage depends upon how much traffic is processed in a unit time. If you have rough idea about how much traffic is likely to be encrypted or how big BGP routing table is ( depends upon BGP implementation ) then you can possibly decide if 2851 would hit 100 % cpu utilization.
If possible you can try with VPN accelerator cards. Those really comes handy when hudge amount of traffic is processsed by IPSEC.
HTH.
Please rate if possible
Thanks
Subodh