To answer this you'd need to know how many prefixes you will be getting from each neighbor or a total number of routes you will have in your 7600, will you be using communities, route-reflection, dampening, # of alternate paths, would you use max-path information, etc.
For example:
With no communities, route-refection, dampening, with a 100K routes you would need 26000468 bytes of memory just for BGP.
400 neighbors seems quite a lot. Is there not a reason that an IGP would work out for your solution?
Hope this helps,
Don
