I'm looking at building my PC and I'm looking at the operating systems now. So my question is what is the difference between 64-bit and 32-bit?
32-bit cannot support over 4GB of memory. 64 bit can support Terrabytes of memory, but it depends on your operating system.
Along with the others: get 64-bit. You can run way more programs, use more than 4gb of memory (if you're intending to use as gaming/basically anything else you need more than 4gb) and you can still run x32 programs. Honestly, you'll probably end up paying more for an x32 operating system since they are becoming very uncommon.