I don't know what the actual cut-off would be, but I have an AMD Athlon x 2 4800 + CPU (overclocked 10%) and 4 Gb  DDR Ram on a 32 bit version of XP Pro, and I don't have a problem with Vue.  Running the same system with 2Gb Ram, I received numerous "out of memory" error messages, which I no longer receive.  My system no longer chokes on Poser imports or large ecosystems.  As a test for stability I kept adding large objects & ecosystems  to Vue to see how much Ram I could "force" it to use (checking task manager), and I had it utilizing over 2.4Gb and it was still stable (though slow).  I honestly don't think Vue under "normal" conditions would ever use 2Gb Ram, but it will attempt to utilize 1Gb with large scenes.   Personally I think 3Gb would be sufficient  for XP.  There are still some problems with "some"  Poser imports that haven't been fixed, but according to Eon they're working with e Frontier to resolve the problem.  Other than the Poser import problem, Vue seems to do everything it claims it can do.