10-20% online at the same time is an extremely high percentage. We tend to max out around 5% and as an intranet site I'd expect our percentages to be higher than a internet site.
I have around a 1000 registrered users, and see 5-10% online at the same time. I have quite a few plugins enabled, including auto refresh of the river and an ajax chat.
A VPS came up short on this, so we moved to a dedicated server with Core i7 and 8GB RAM - now everything runs smooth and fast.
We all hope to get busier, so this is a VERY helpful thread! Thanks for the input from the guys with experience.