Ashley,
You ask quite a few questions there let me see if I can answer them in order.
1. Windows 7 & 64 Bit IMHO I would spend the extra & up the RAM as you will notice a difference in performance. I made the leap & still have Computers in this Office that are running on lower specs & I really notice the difference when I go back to one of the older specked machines.
2. ArchiCAD 14 comes in both 64 & 32 bit configurations & the install is determined by your OS being either 32 or 64 Bit. Obviously in your present config XP is 32 Bit but if upgrade to Windows 7 then you have the option of either 32 or 64 Bit & at time of installation you should determine which best suits your needs. IMHO I would suggest 64 as it is the move forward.
3. As eluded to in the previous answer it doesn't use both 32 & 64 Bit it is installed as per your operating system & makes the intuitive choice without your need to intervene at installation.
Hope this helps