I also posted this in the Software/XP forum but have yet to get a reply.
That's a lot less heavily trafficked than the CPU forum is.
Does Having the OS take advantage 64 Bit hardware make a Difference..??? Does a 64 bit OS run faster than a 32 bit on a 64 bit CPU and MoBo..??
That all depends. A 64-bit OS always has three advantages over a 32-bit OS:
1. It lets you use more than ~3.3 GB RAM.
2. The code can be compiled with much better optimizations than generic i686 code can be without risking incompatibility with certain older processors. SSE2 particularly comes to mind here.
3. Integers are by default 8 bytes rather than four. This makes some math applications more efficient and gets rid of the need to explicitly compile programs with large file support to make >2 GB files. This isn't really an issue unless you code...and then it can be a real help.
It also always has one disadvantage: 64-bit code will always use more RAM than 32-bit code due to integers and pointers being 8 bytes rather than 4. Many programs use a full 8-byte integer to store single-digit numbers (e.g. loop counters) and less than 1/100 millionth of the storage capacity of the variable is used. This leads to more bloat.
Other than that, your experiences will vary OS to OS and program to program. Hardware support varies widely by OS for 64-bit support. Linux has 99.9999% of the same hardware and software supported in x86_64 as it does in 32-bit x86, but 64-bit builds of Windows have far fewer drivers and programs available than the 32-bit builds. Solaris has pretty decent x86_64 support, and I hear the BSDs do as well.
I know the applications that are 32 bit will/should run about the same, I think.? I don't think the 64 bit version of XP cost any more than the 32 bit version. I am considering using an old 32 bit W2k Pro for my new build, but will probably get XP pro for comparison.
32-bit applications will run just fine on most x86_64 OSes. The only thing that's required is multilib support so that a 32-bit program keeps its 32-bit libraries separate from 64-bit libraries. Windows has this and so do most UNIX-type OSes, at least by default. You can have a purely 64-bit system on a UNIX-type OS if you install it without 32-bit library support (which means no "lib32" folder, analogous to no "i386" folder in Windows.)
Your suggestion to use Windows 2000 is a good one, provided that you do not want to run more than 3 GB of RAM and have hardware that has Windows 2000 drivers. Most stuff that will run on XP will work on 2000 as well, but a lot of hardware that's sold for use with Vista by OEMs (particularly laptops) do not have XP or 2000 drivers. Caveat emptor.