You could try to directly in Windows 7 and if that didn't work, change to the Compatibility Mode of the Windows version the drive were created for. Shouldn't be any problem!
It's upon your machine, see, every machine is a "similar" to another ones, different scenarios. The answer is to test by yourself.
No, try Win7. I am a PC technician and had the opportunity to upgrade / downgrade machines, depending on the case and I can assure you chances are good for your PC to run just fine. Try it. If not, then go down to Vista.
If you're going to stay with 2GB RAM, 64 bit is contraindicated. 64 bit will run with 2GB but it's much happier with at least 4GB. You just won't get much of 64 bit's advantages with 2GB. RAM is cheap, if you system will handle it and you want 64 bit advantage, buy additional RAM, otherwise stay with 32 bit.