Capacity planning for the Edge tier is difficult to generalize. You should work with the hardware and software vendors who are supplying equipment for your deployment to develop a capacity plan. Nevertheless, you should implement the Realtime Blackhole List (RBL) at your site at the Edge tier. The RBL is a list of IP addresses whose owners refuse to stop the proliferation of spam.
Design the Edge tier for minimal latency (less that one millisecond through the entire Edge tier).
Use load balancing algorithms that are load-aware by CPU utilization or by the number of active connections. Round-robin is not an acceptable load-balancing model. With the exception of MTAs (stateless), use sticky-bit load balancing.