Slow Site Speed

The speed of my WPMU/Buddypress sites is abysmal. We have tried a few things to speed them up but with little affect...we got some advice to do APC(Alternative PHP Cache) but couldn't get it to work on our server. The sites are running on a Hostgator VPS. I downloaded PageSpeed and YSlow and both are giving my sites a grade in the low 60's and I have no clue how to execute the list of things they give you to speed up the sites.
Speed is not the only issue I believe the Autoblog plugin is sucking up the memory and crashing our server at least once a day.
I have 50 wpmu/buddypress sites I would like to get up but with the issues above it is just not happening...does anyone have any advice or suggestions to fix these issues or can you point me in the direction to someone that can help us that won't charge and arm and a leg.