Yipes here goes...
I'm thinking of buillding a multi-site, 1 site owner with variations of content on 3 sites total.
I really don't have a complaint about building 3 op sites. I was just thinking ongoing site maintenance and upgrades would be easier.
The site will be on a shared server with plenty of memory and bandwidth.
I have a couple of OP working on shared servers now, and haven't had problems. That may be because I delete news feed articles after 2 weeks, and use poormans cron with a 3 hour cycle.
There is no way of telling what the traffic on these particular sites will be, because I'm unfamiliar with the content and how popular it will be.
I will use a filter on all the news feeds for some very specific keywords, so the feeds will not be pumping a ton of information to the site like a newspaper or magazine site.
NOTE: I appreciate this groups very much, because everyone, including the developers. Everyone is so involved and issues are dealt with quickly.
Yeah, I realize this posting may be considered lame. I don't care for a bunch of guestimates... just a few thoughts, if you have some.

Comments
i am doing 3 sites...
with OP2.0 now... seems ok.. but just starting.
I did notice that the ardi theme is not compatible with drupal core 6.16...
and have tried a couple subtheme tricks... no luck so far.