Do I need a 32-bit or 64-bit version?

64-bit versions will only run on 64-bit computers, but 32-bit will run on both. If you have a specific need for a 64-bit version (including the use of more than 3-4 GB RAM), then that would be the right option for you, otherwise we recommend 32-bit versions.

There is no real benefit to running a system compiled specifically for 64-bit computers. Binaries and libraries are 50% larger than their 32-bit counterparts. The same is true for the amount of RAM that is needed for 64-bit versions. The running speed of a 64-bit system is actually slower than a 32-bit system on the same hardware. So again there is no upside to running a 64-bit system. But a 32-bit system will run faster on a 64-bit CPU.

Also, there is still a lot of 32-bit only software in use, so a 32-bit operating system would be needed to run this. More details can be found at Wikipedia