Do you have a 64-bit OS? If you don't, it doesn't matter if you have a 64-bit CPU, the 64-bit programs will NOT run. 64-bit refers to not only the instructions that your CPU can execute, but those that your OS can ask the CPU to execute. If the OS is 32-bit it can't do anything with 64-bit instructions, so the program won't run.
And implied in that (yes?) is: why then have a 64bit system?
No.
The 64bit CPUs (athlon64, core2
TM, even newer P4s) can run 32 bit stuff faster than the old 32bit CPUs.
95% (total guess) of the systems sold today have 64 bit CPUs.
The only reason not to get a 64 bit CPU system today is $, but you lose a lot of performance for very little cash saving.
Simile: 64 bit is like towing with a car.
Some cars have a towing
capacity (64 bit), some don't (32 bit). But to tow something (run 64 mame), the car also needs a tow hitch (64 bit OS). But even if you can't get (or don't want) that tow hitch, that's no reason not to get a car just because it has the
ability to tow. And even if you had that tow hitch, you still don't have to tow anything.
In only words, even if you aren't going to tow anything, don't rule out the Mustang just because it has a towing capacity; same with CPUs: even if you aren't going to run 64 bit apps, don't rule out 64 bit CPUs.
FWIW, it used to matter back with 16 bit vs 32 bit CPUs (16 bit apps on penitum vs pentiumPro), but that was because intel intentionally
dropped 16 bit speed. The earlier 386, 486 & pentium CPUs didn't have that problem, and neither did the later PenII. And none of 64 bit cpus have dropped 32 bit support yet (and won't as long as people keep buying vista & XP instead of vista64 or XP64).