Just think back to Windows XP 64-bit? Who ran it? Who even knew (cared) a 64 bit version existed? Definetly not the hardware manufactures, as there was no hardware support. Then, when Vista came and the drivers were there (except for my HP Laserjet 1000 -- thanks HP!!!!) there was a choice. Windows 7 is building on amazing, stable 64-bit Vista code, and since moving to it, I have had no problems with any software; why would I run 32-bit and cripple the capabilities of my system.
When I upped my memory from 2GB to 4GB (for a whopping $50) the system ate that memory up and sped my system up considerably. Mostly in the speed in which applications started up, switched around, and obviously on how many I can have running at once. With a 32-bit OS, going from 2GB to 4GB would not have been as beneficial as you would have to trick the OS into using the extra memory -- inefficient.
Now, it is not all rainbows and fluffy bunnies: If you run PCI cards (in my case I had two -- Audigy 2ZS and a Hauppauge TV-Tuner card) you cannot use them with a 64-bit OS with more than 3GB of memory. The problem is with the 32-bit PCI interface and timings. I am not a hardware expert, by any means, so I don't know the exact details as to why it hampers the PCI bus, but I just know it does (my tv-tuner card and sound stuttered horribly). The best solution is to have all PCIe cards which doesn't experience that bottleneck.
Keep that in mind when making the decision to upgrade to 64-bit.
Also, if you have 32-bit software, you could always run Virtual PC and install 32-bit XP or Vista/7 and run the application in there, if plausible.
Sliver
Sliver