I strongly recommend staying away from Wordpress MU and a centralized DB. We made that mistake a few years age and it made selling off pieces of the network very difficult.
I also recommend a cache plugin like Super Cache. We are currently running Quick Cache on 200 blogs and it works great. The only problem with caching plugins is, if you run a link trade script traders won't see their links until your sites caches so you will get a ton of emails. FUN FUN FUN
Speaking of links check out our network in SIG for some (delayed by one hour) hard links
Thanks