Unfortunatly, Invision boards are often very resource intensive, and are hard to keep running. I would look for another board, the smartest is to go with vBulletin (http://www.vbulletin.com), which will cost you some money.
The only thing you can do is limit the boards, and make it less graphic intensive..
No problems with IPB here either. You have only ever had 58 people online at once? How can that be resource intensive? I run 150+ users on a shared server and never have any issues. 733 web sites on the same server might be affecting resources on your server.
I know whois.sc isn't always accurate, but I'm sure that isn't far off.