You should really get 64-bit because with what you're going to be doing it will be useful to have more than 4gb of memory. If you want to use more than 3.5 to 4gb of memory, you will need to use a 64-bit OS so all of the gigs are registered rather than only 3.5 or 4 (that's the max a 32-bit OS can read).
To run 64-bit operating systems you need to get the 64-bit editions/versions of it. So, Vista 64-bit edition, etc. I don't think that you can buy XP at retailers anymore... I might be wrong, but that's what I heard a few years ago (that XP's is being taken off the shelves, and that Vista is in power). <I might be wrong> Google it.
You can run most 32-bit applications on a 64-bit EXCEPT hardware drivers.
I found this handy website: http://windowshelp.microsoft.com/Windows/en-US/help/41531554-d5ef-4f2c-8fb9-149bdc5c8a701033.mspx
You should check it out.
I would like to build a system that can be used for gaming, video editing, Photoshop, and some CAD. Questions:
To get a good system, do I have to move up to 64 bit?
What is the most durable system I can make with 32 bit?
[My main issue with the two are that many of my programs run on 32 bit and I really don't want to purchase the 64 bit versions, who knows if they're even available.]
Can I downgrade the system (windows XP Pro) efficiently and still get the most out of it, that is, if it was intended to run 64 bit OS?
I've seen a few barebones kits on tiger direct and newegg, are any of those worth getting?
Thanks for the help.

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