Server load

I have a resellers account on a shared server. I recently received a notice saying I am overloading he server and need to move to a dedicated server. I am running several installs of WP for clients, but my main concern is my Oxwall installs. I have three of these on my main account and realize the WPMU may be a better way to go.

Anyone have any advise on what is the best way to reduce server load and still build private sites for my groups?

What should I expect for server requirements and do I need to go dedicated already?
Will a dedicated server really resolve this?

Thanks for the help
Richard