That question is almost impossible to answer, because it would depned on many other factors as well.
You may be able to build quite a large scene, but when you go to render it, you may run out of memory becuase of the number of lights and shaders you are using, plus the number of transparency maps as well. What are you having problems with, is it rendering?
If so, you could try hiding some of the scene items, rendering each one seperately, and compositing them in Photoshop or similar. The Gen 4 models are all around 70,000 polys, and Genesis is onlly about 19,000 in its base state before sub-division. For gaming, models are usually well under that, around 7,000 fully clothed.
If you look in Window > Tabs > Scene Info, that will give you an idea of how many polys are in each item. What items in particular were you looking at that are very large? When you have your scene set up the way you want, you can always hide any surfaces which are not in the camera view, and that would help. The only real cure is more RAM, and a 64 bit system.