Quote -  But doesn't all those saved renders take up memory? I always lower mine to 2 for that reason. Or is that only ther undo levels that take up memory?

I think that all the previous renders are saved on the harddisk. So they will not
use any of your RAM, only disk space.

This is the path pp2010 holds them for me:
C:UsersMeAppDataRoamingPoser Pro2.0RenderCache "timestamp".exr
Poser save's them in .exr format.

I have set my value to 100. So its easy to display former renders, that i am not saving in
my ..render directory for perma storing.
Im using this feature almost evry time when i work with poser, to compare differences
in my renders, when i changed things in the scene.

btw: did you notice the WindowRecent Renders Menu.
Via this you get a preview of all your cached renders (pp2010)
(and of course, using this prievie cost's a bit memory, :)