At what point do Elgg sites typically have to go from shared to dedicated? 

I've been getting mixed messages from my hosting company.  Not sure if there's a problem with my site or if it just gets too much usage for shared.  I regularly have between 20-30 users online at a time and I've not seen it drop into single digits.

I disabled all plugins except profile, messages, uservalidationbyemail, and sociable_theme for 24 hours.  This made an impact on the site usage, but the server resource usage was still too high.

Has anyone experienced similar problems and what was your solution?