How many (if any) full BGP feeds should a 3845 with 256M memory normally be
expected to take?
Less than one? 
Agree. Our 2821 with 2 full views running 12.4 mainline and 768MB ram
has 427 free. So using 340 for OS and tables...
Chuck
3845 is plenty powerful, its the equivalent of an NPE-400 basically, but you should plan on 512MB minimum with full BGP, so that you make sure you have room for other things as well on the router.
Its not linear of course, since there is alot of redundant information with multiple BGP feeds, so you won't need another 512MB per peer.
Brian