64 bit windows recognizes more than 3 gb of memory and only requires a 64 bit cpu, which all newer ones are. The new windows 7 professional or ultimate can work with old files from windows xp with a cpu that has "virtualization" and a board to support it. But these versions of windows are $320 as stand alone programs, too steep for my budget.
I feel like a fool asking these questions... I'm more focused on the Network Admin side of things. What was the first Intel 64 bit CPU? I know AMD's was the Athlon 64. What are some advantages and drawbacks of using 64 bit Windows over 32 bit? What about using Windows Vista x64, will it work with all my 32 bit and older programs?
The earliest ones for home users were the late model P4's I believe.
As for software, 99% will work flawlessly, as the OS handles the 32->64 conversion. That being said, all drivers MUST be 64-bit, so lack of device drivers can cause problems. (Example: The retail version of Splinter Cell:Chaos Theory will not run because the drivers for the copy protection are 32-bit, although the game itself works fine).
At this stage, unless you are running really old hardware, 64-bit is the way to go.
I should make a quick note: As of Vista, all 16-bit apps will not run in windows, regardless of what version you use.