Windows 7 32 bit or 64 bit and CAD 2013 32 bit or 64 bit

I have a little question.

I now use Windows XP sp3 32 bit + AutoCad 2013 32 bit and i have some memory limitations because i work in multi autocad windows, i prefer to work like that.

We are going to have windows 7 soon and i would like to ask for the 64 bit OS hoping to add 8 gigs of memory to my computer (i have 3 right now)

01: We have Autocad 32 bit DVD, can we just exchange 32 bit and 64 bit using the same auth codes and keys we already have? or downlod 64 bit version and add our 32 bit auth. codes?

02: If we have to buy 64 bit version (We are not going to buy it):

- Shoud i ask to install windows 7 32 bit and use Autocad 32 bit to get better stability ?

- Or should i ask to install windows 7 64 bit, add some Ram and use Autocad 32 bit ? Is it going to be stable?

I would like to get some great advice on that please.

Thanks.