BuddyPress.org

256 is pretty big, what you set it to depends on what is available, and also bear in mind what the limit is actually put in place for which is to prevent a script crashing your server, if you set the limit to all available memory you would be in trouble if a badly written script looped and started to use up all your system memory. If you can try increasing the memory allocation in chunks testing each time to see if that is the problem and at what point or extra amount everything settles down and works, then add a bit of headroom.