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#11
It's basically going to come down to the programs you use and whether or not they're optimized for a 64-bit OS. You'll have to do some research on the programs you use or may wish to use to see if they're optimized for 64-bit.
That's the shortened answer to a rather complicated question.
The really short answer is memory. If you have or desire to use more than 4gig of memory, then go with a 64-bit OS.
To be honest a lot of folks using 64-bit OS' really don't need to, outside of memory requirements/wants. And in my opinion, if you have no more than 4gig of memory, you could get away with using a 32-bit OS, provided your video doesn't take too much from your system memory, but now we getting into other areas.
At any rate you can read this article here to get a firmer grasp of 64-bit - 64-bit: More than just the RAM - Vista Forums