Actually, the new rendering model is significantly more memory intensive as it is based on something akin to buffering each individually rendered window (though it will try to buffer in video memory before system memory). The previous model is very optimized for low memory consumption and in general simply redraws where needed instead of using buffered or cached data (though there are some previsions to do this, they aren't the default or the typical case).
There are disadvantages to the old model when it comes to simply not being able to do things like the kind of visual effects presented by the graphics system when in WDDM mode. If you're worried about it's memory usage and don't care for the extra visual effects, then I'd recommend turning off Aero.
You cannot, however, have a free lunch - if you want the flashy visual effects, then you'll need to be willing to sacrifice video memory (and eventually RAM after that fills) for buffering for WDDM/Aero.
