I am doing some proof of concept for an MPLS data centre and did something I have not normally done. I used my MacBook Pro to run Dynamips / Dynagen. I am very pleased with 9 3640 routers running Enterprise Plus, and 2 2611 Routers with IP and running less than 60% CPU across both cores
This means that I have still have enough resources to run quite few other programs including Parallels (although it does run a little slow). I have an older MacBook Pro, but upgraded to to 4 GB RAM. I found that the 2 x 2GB RAM modules run faster than 1 x 2Gb (as you might expect) but its a shame that only 3GB can be used or I could have quite a few more.
I won’t repeat what other people have said about Dynamips, but running it on OS X Leopard is very fine indeed.