Hi
I recently got a new 8 core machine for work and 8 GB of ram, and I finally installed ubuntu linux 64 bit, to take advantage of all the system.
I had before a 4 cores machine with 4 gb or ram and always worked with 32 bit, cause, yes, 64 bit requires more memory per task, some say is double the ram, so now that I have enough I went 64 and yes I'm having a nice time

A couple of things I should mention, 64 bit windows is not that good, seems like they just made the system 64 and NOT optimized it to really work like that, for this kinds of really hard stuff I will always choose to do them in linux, like rendering simulation, compositing, cloth, etc, linux is much more stable, crash less and once you get windows out of your system, actually friendlier that windows, and now with the latest WINE you can run almost anything inside linux even ZBrush.
One other thing, linux has PAE kernels, this is simply put, a 32 bit kernel that supports more ram that the regular 32 bit, I think it goes up to 64gb of ram, so if you only need to use more ram, you can use that, centOS linux installs that automatically if you have 4 or more gb or ram.
To finish I will say, go 64 if you have enough ram and more importantly if you are gonna make your machine really really work hard, if not you might not see almost any difference.
Hope this helps you and if you haven't, try linux, make your computer work like it supposed to
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ubuntu.com]