If you go with 32 Bit it wont matter which OS you use. They are all limited by the 4GB limit. It is a limit with the 32 Bit kernel within Windows, OS X and Linux. So no matter what OS you use if it is 32 Bit it will only use 3GB to 3.5GB of RAM depending on the Video card you use.
Now if you use 64 Bit then it wont matter. XP and Vista both will use the amount of RAM you throw at them. But the limit is 8GB. As far as i remember. At that point i would say you would be better off with Vista. But that is just my opinion.