I'm absolutely convinced that I observe a significant improvement in performance -- even when many applications are still 32-bit. All of the various services and bundled apps with the 64-bit OS run faster. I've seen bottlenecks opened wide, whereas XP 32-bit was choking my system -- or running significantly slower. For instance, I don't have problems with the occasional stutter in video playback that would occur with XP MCE. My TV application would halt momentarily when I'd open my e-mail browser. There may be other aspects or explanations to that shortcoming, but it doesn't occur at all with my 64-bit VISTA OS. And that's for comparing Media Center for each OS on the same quad-core system.
So far, there've been two software applications I've needed to upgrade for failure to run well in the 64-bit OS. And I can no longer use my 13-year-old inkjet printer -- which had taken a licking and kept on ticking -- a workhorse. I can still get the cartridges for that printer and get them easily. But it has a parallel-centronics interface. And the worst of it was the total failure of the driver bundled in VISTA-64 for that particular printer.
On the printer angle, it cost me $750 in 1996 dollars. The replacement does everything as well or better than the antique -- and I picked it up last week for $220 in 2009 dollars.
I don't think I'll ever use a 64-bit OS again, at least -- for a workstation machine. WHS 32-bit is fine -- for what it does. But on the client/workstation end? The only reasons to avoid a 64-bit OS are such as I've shown here, and -- really -- it's about time to upgrade a couple $60 software packages and chuck a 13-year-old printer . . . . isn't it?