I would suggest you go for the safer option of the 32 bit version.

Plug in all your hardware, e.g. printer, scanner, cam, etc. then download and run the Vista Upgrade Advisor from here http://www.microsoft.com/windows/products/windowsvista/buyorupgrade/upgradeadvisor.mspx

This will tell you what software and hardware will or won't work.

It's always best to clean install rather than upgrade.

Lastly, make sure you have off-line copies of all your personal data, e.g. pics, music, videos and the like before installing.