I personally believe that 64-bit computing will start to go mainstream several months after Longhorn goes RTM next year. Windows XP x64 is a pretty solid OS for the most part, but driver availability is not yet up to the mark and there are still very few "killer" 64-bit applications for regular users.
Since Microsoft will be releasing 32 and 64-bit versions of Longhorn at the same time, hardware manufacturers are much more likely to develop drivers for the 64-bit OS at the same time that they do for the 32-bit version. System builders like Dell, HP etc will also begin to offer more x64-based systems pre-installed with the 64-bit version of LH. That leaves us with the question of killer 64-bit apps, and I think more of those will start to show up as the adoption rate of x64 processors goes up.
With that said, x64 is definitely the future of computing, and it'll take off sooner or later, so for someone purchasing a new system at this point, it's a good idea to go with a 64-bit processor if cost is not much of a problem. After all, you don't lose anything by taking that route since 32-bit OS's and applications run just fine on these systems too. But when 64-bit computing eventually becomes mainstream, you'll have a system that's ready to roll.