I completely agree w/ you on this one..  A lot of my scene that are not pinups are pretty massive in size (height and witdh), lots of details, and lots of figures..   This one  contains  10 different figures,  about  100 tombstones,  one big ass  oak tree, many props, lots and lots of ground cover and much much more.. lol..  Now, my machine had no problem setting it all up like this but not even close to a preview render.. lol...  So what I did was lock the camera where I wanted it and set up parts of the scene as separate renders where I wanted the part to go..   Then once I got all of my parts rendered put them all together in photoshop..  I had about 10 separate renders maybe a few more..   I ended up w/ 3 separate psd files that had certain effects..  Then I merged them all together into one for the final piece..

http://www.renderosity.com/mod/gallery/index.php?image_id=1551507&member