One Wordpress install (multisite) or multiple installs?

I manage a server for a client who has several Wordpress blogs. They all precariously balance on PHP/FastCGI/nginx and it all works at the moment, just using a little more RAM than I'd normally be comfortable with.

Since setting this all up, Wordpress 3 came out and (with a plugin to maintain domain mapping) allows me to import all the blogs into one central blog. So I know I can achieve a desirable operating outcome... I'm just not sure if I want to sink hours into doing it.

The client now wants to add a couple more blogs to the server. We're clearly at a crossroads where it would make sense to act now if I'm going to go down the multisite road.

I don't really care about the clear administrative benefits rather the performance and the amount of free RAM.

Do you think I would be correct in assuming one codebase in RAM would cost less than having half-a-dozen single sites? Are there any other performance pros (or cons) that I'm missing?

The short answer is you are better off going with a single Wordpress 3 install rather than maintaining separate databases, servers, configs, etc. for the sites individually. The key piece of information to understand here is that Wordpress 3 contains most of the core code that runs the multi-million site/blog wordpress.com, minus some obvious mass-scaling optimizations.

The only challenge you may run into in the future is if one of your sites wants to branch off / go somewhere else - you will need to define a migration process (which ultimately should not be that difficult :) ).