Hi Fady,
You can do several things as I see it.
You can run EIGRP between your branches and the 3528 router, and the run RIP or OSPF between the PIX and the 3828 router. Redistributing EIGRP into the routing protocol you choose.
You can run EIGRP on all equipment, and use a GRE tunnel to get it throug the PIX.
You can also use OSPF on the entire network and put the PIX into the OSPF routing processes.
Kind regards