I'm all for purging old members, but I for one don't like pruning forum content. I'd go say purge items that are worthless (like our "tempting" thread here at central) but keep quality information.
Honestly, standard hosting is outside of the scope for a forum of your size, dedicated resources are likely a nessessity at some point in every communities life...
And why backup 4 times a day? that can be causing a LOT of lag... Honestly, is your data so important that it exceeds a need of a WEEKLY backup schedual?