We Should All Live in a 64-bit Submarine

"Long answer: 64-bit, but you may not see much real difference. Before explaining why, there is an important contraindication to be aware of: if you use any 16-bit Windows applications or DOS applications, you'll have to either stick with 32-bit Windows, or run those applications in a virtual machine (or, for DOS programs, an environment such as DOSBox). 64-bit Windows supports 64-bit and 32-bit applications, but 16-bit ones are consigned to the trash can of history."

Mac users that have anything remotely current will boast that they have been using 64-bit software for quite some time now, and that is true.  The Windows world is a bit murker.  For the longest time, 32-bit Windows XP was good enough.  One should note that Windows XP is a decade old.  Sure, Microsoft came out with a 64-bit version, it it was plagued with problems at the beginning, and driver support was a laughable dream. 

Things have changed over the past few years.  While there are still some 32-bit Windows 7 (or Vista) users out there, and there are even some 32-bit CPUs (Shock!  Gasp!) being circulated, the majority of computers I am seeing being sold now come with 64-bit Windows 7 Compu-Global-Hyper-Mega-Net Edition.  I also find that most any hardware you plug in now has no issues with drivers.  The 64-bit world is the future.  The main benefit is the ability to go above the 3GB RAM barrier, but with support the way it is, unless you have some really old legacy application of some sort, there is no reason NOT to go with 64-bit computing.