hi
would suggest to chek out with static routes in all your boxes.
Just the default of your 7200 towards net and internal network pointing towards your 2800 ethernet ip.
and in your 2800 default pointing towards 7200 and the internal networks pointing towards your pix interface.
we had similar kinda setup done for our customer except a seperate VPN router in place.
since you have less no of boxes in place i suggest for static if u still want to go for some igp then check our for ospf which can also help u out .
do revert if u need more details on the same..
regds