I agree with both Michael and Genner
If you know you'll only use 2 servers, put the strongest as the webserver -- Moodle uses more CPU cycles and memory there. On the other hand, if you want to prepare for "transparent" growth, have everything ready for a cluster, with the strongest server as the DB server.
One thing that is important: the webservers want a lot of RAM; not necessarily the fastest CPU, but a lot of RAM. If you have many users on dial-up or other slow connections, a reverse-proxy in front helps heaps too.
By the way, it's odd to hear 30K students classified as mid-range. How many students have you got, Michael
? I don't really worry, that cluster will be growing in usage, to (I think) ~90K users towards the end of 2005. We'll be learning *more* about Moodle scalability, and I may earn a "top-range" label for the cluster