Windows 7: 2 computers 1 OS 64-bit 32-bit?

I'm Have a custom tower atm that i built. it has windows XP installed by an update disc. Due to my computers age im building a newer computer running a Phenom II X2. The problem is I'm not sure how the OS's work, haven't messed with them sense XP. I want to install Windows 7 on the new computer in 64-bit. however I also want to install it on the older one in 32-bit, is there a disc that has both or do i have to buy one or the other. and is using the windows 7 disc on 2 different computers that belong to me against the terms of use? ty for any and all help =]

You can have your choice of either 64 or 32 bit for each license bought.

If you want to try each one or both before deciding, you have 30 days to activate one or the other at Computer>Properties activation link. Just don't insert your Product key upfront when installing and uncheck box to Activate online.