I'm still using my 700Mhz AMD, with 756MBs of RAM.

RAM is the bottlecap that forms when you measure how big your scene is(with all it's stuff added,) plus how big an image you want to render.  A Lack of RAM is why a render will fail.  The CPU, once your RAM requirement has been satisfied, dictates how fast the render will complete.

One big thing to watch out for, when building a scene, are morphs  Before a big render, or when creating a scene with lots of people; you only need one morph per Figure Part.  If you look at th image, in my gallery, that my avatar was made from you will see V3 scene that used to be 250MBs; without any props, just V3.  By just removing unused morphs(those set at "0") I trimmed that scene down to about 65MBs.  Then when I consolidated all of her morphs, to one per part, I came out with like a 6MBs Poser scene. I'm not sure how to do this in D|S, but Poser has a bunch of free Python Scripts to help i.e. SVDL's RemoveMorphs.py, and SpawnFigure.  BTW(SPawning is where consolidation happens.  Poser has a function to Spawn, it's just a little labor intensive(for a whole figure)  Ockhams zeromorphs.py, is also a big help.  Ohh...Morph Manager is another free utility to get through this, which I think we can use here with D|S?

RHaseltine was correct WIndows can access 2GBs of RAM, per application; and it-self is counted as an application.  I'm not so sure about his 4GB perspectus though.  I've heard of an XP Pro 4GB limit, but this was in reference to how much virtual memory is allowed per disk.  I do'nt know of too many motherboards that will even hold more than 4GBs of RAM, but as far as I've known there was'nt a limit to the number of applications you could open, at 2GBs a pop?  And as far as I know a Vidoe Card would need to have Virtual Textures, in order to occupy Physical Memory.