I personally run the 64-bit version of Windows XP and I am not going back to 32-bit. There is a few down sides with this operating system as its tough to find drivers for some stuff from minor hardware providers, the ones where I have had the most problems is C-Media chip based Sound Cards. Now, any major hardware providers have 64-bit drivers, most now getting out of beta. I will be staying with 64-bit when Vista is released for no other reason then being able to run 64-bit Far Cry (and Far Cry 2/Crysis when it gets released) and Half-Life 2 64-bit. There is not any HUGE advantage to these two as far as performance, its the eye candy additions that makes me happy, example, in 64-bit Far Cry, the visual range was bumped up. Its especially noticeable when using the Binoculars, its hard to explain but if you see it, you will be in awww.
As far as 64-bit drivers for Vista is concerned, I wouldnt worry about it too much, there will be plenty.
Two major reasons there is not an abudance of 64-bit drivers now is, 1, when Microsoft released windows xp x64, they modified the system dll's and other system wide resources a tiny bit so you cant, or couldnt, just re-compile a 32-bit driver to 64-bit (Visual Studio 2005 lets you do this fairly easy but you have to be using x64 windows to compile in 64-bit), and 2, outside of server use, 64-bit CPU's were not nearly as abundant as they are today so the 64-bit Windows XP operating system is not widely used with system builders and its hard to migrate support w/o drivers there.
With the first problem, Microsoft is building the 32-bit and 64-bit Vista hand-in-hand, at the same time, to ensure easy programming/compiling between the two, so driver creators need only to set the compiler's to compile it in 64-bit and bingo, you have one of each. Also, somewhere, I could swear I read that Microsoft was trying to make it so the 64-bit version can use 32-bit drivers, I doubt it would be 100% reliable to do that but even so, you wouldnt be using the full capability of the 64-bit system if you are using 32-bit drivers. This may have just been a rumor mind you.
With the 2nd one, you can now buy a 64-bit CPU for $50 and a compatible mobo for the same. 64-bit is here to stay (atleast until 128-bit is released
) and it is available from both major CPU vendors, across all lines of processors, Celeron/Semperon, Pentium/Athlon, and Xeon/Opteron, ranging from $50 to $1000+. And now that there has been a few programs and games released to truely take advantage of 64-bit, people, like myself, are beginning to see the gains in 64 over 32 bit which you can only get using a 64-bit operating system.
IMO, the 32-bit version of windows vista is only being made for backward compatibility with older systems and may not be used by major hardware manufaturers (unless the CPU is 32-bit of course). I can see Dell and HP openly providing a choice between the two versions. Right now, unless you SPECIAL SPECIAL order hardware, like over the phone, no one gives the option between 32 and 64 bit windows xp, which plays a part with problem 1.
So some parts, it comes down to the, which came first, the chicken or the egg, problem, w/o 64-bit hardware drivers, there is no use for 64-bit windows and w/o the operating system, why take the time to create the driver anyway. But I could see a major migration from 32-bit OS to 64-bit OS within the next year or so.
So.... what I would do, see if you can find a 64-bit drivers for either Windows XP x64 Pro Edition or Vista x64 (probably beta driver only) for your hardware and if you can find them for your entire system, go for 64-bit Vista, heck, ask your buddy for a x64 version of xp and see it for yourself.