Same key for 32bit and 64bit XP

Hi there. Just a couple questions about XP 32bit and 64bit.
I'm currently running 32bit XP Home but I would like 64bit Pro so that I can add more RAM to my computer.
The thing is that I don't really wan't to have to pay a couple of hundred dollars to buy 64bit so I would like to to know if there was another way.

I can get the 64bit disk so that isn't a problem, the thing is the key. I have an old Dell computer sitting in storage with a 32bit Pro key on the side of it. The computer has been used for office work for a while so the key has been used.
I was wondering if I could use the Dell key, which is 32bit, to install a 64bit version on my current PC. Would this work or is the Dell key now used up, and do I need a 64bit key for a 64bit install or does the 32bit key still work.

Thanks in advance.