A few things....

1) System resources depend on your environment. For example, many production sites run something called an op-code cache. This helps with performance in a big way. If you use an op-code cache there can be a large benefit for memory and speed for a single install.

2) If you are going to do upgrades you should be doing them and testing them on a test server and then migrate those changes to your production environment. There are tools to handle upgrading multi-sites and there are strategies for upgrading one of them at a time.

I run multi-sites for mine and I'm able to handle the updates to them one at a time.

3) If you are to have large files on your server you might consider using a cdn. Your web server can only serve so many files. Large files take up these channels and can greatly reduce the number of visitors that can come to your site. Check out something like simplecdn.

Thanks for the info Matt.

I plan on trying your tutorial on installing APC.

I do use a test environment to test updates before pushing them live to the production sites. My concern was having to update all our sites at once. I will have to look for the tools to hasten the multisite upgrades.

We currently use a CDN for our free online videos. I am looking into the options for using a CDN for Ubercart online products.

Thanks for your thoughts.

Phill