The catch is that there are very, very, very few applications written to take advantage of 64-bit processing.
The reason that Itanium was such a flop was that it ran 32-bit apps slower than a 32-bit processor.
For the average personal PC user, there is no real benefit from 64-bit processing. Where they are useful tends to be in memory-intensive, compute-intensive applications.
As and when games can demonstrably take advantage of 64-bit computing, there will be a surge in take-up. Historically, games have been a significant driving force behind PC (as opposed to server) development - I believe the same will be true again.
I figure that there's a couple of years where 32-bit systems will continue to get faster, but will be forced to sell cheaper so that 64-bit systems can be priced competitively. I'd go for a fast 32 at a fraction of the cost of the 64.
SD