There are a lot of Windows OS (Operating System) versions. Have a look here -->
http://en.wikipedia.org/wiki/List_of_Microsoft_Windows_versions
But when it comes to 32bit vs 64bit architecture, the license key is interchangeable as far as I know as long as you stick to the same Windows version (Home Basic, Home Premium, Pro etc...). And you can only use either or, not both.
So if your PC has an OEM sticker on it with a license key then this is usually valid for both 32 and 64bit version of that particular Windows OS (Just confirm this if you're using Windows XP as I'm under the impression XP 64bit was a separate release - but XP and 3D rendering won't go well together for the latest software anyway...). So then just download the 64bit version of that particular OS if you can't find your discs. Remember to backup everything before you install the new OS over the old.
If you have to buy a new MS license and DVD's ensure one of the discs say 64bit on it to save you the hassle.
http://community.spiceworks.com/topic/273553-windows-8-32bit-key-will-work-on-the-64bit-version
As for choosing a particular OS type if you're buying new, well that's another debate

I'd go with Windows 8.1 (Standard, not Pro) if it were me, because I'm one of the few who aren't bothered by lack of a start menu as I'm not pathetic that way