How much RAM you got in there, and can it be upgraded, and if so, by how much?

If you've got less than ~3.2 GB of RAM and it can't go higher, 32 bit 7 is a no-brainer. If you've got more RAM (or can upgrade to more) then you'd need a 64-bit OS to take advantage of it. However 7 is better anyway so if you've only got a little more RAM it might still be the way to go.