What Do You Need To Watch For In A Linux SMP System?

thefin asks: "My research group has finally received funding (~$200K) for a single SMP box. I've looked over the offerings from SUN, IBM and Compaq. I was wondering what others think of the SMP offerings and in particular, opinions of Linux as a SMP OS. What should we know before purchasing such a machine? What should we look for and what should we avoid? We will be using the box for large, individual based ecological modeling efforts. These models are CPU intensive, and make heavy use of inter-node communications. In particular, inter-node communication can be a serious bottle-neck in our models."