How much benefit will that Athlon64 or P4 EM64T net you when used with fully optimized software, designed for 64bit registers? That's a pretty ambiguous question when asked straight out and depends on a lot of things – Though in the same fashion that the switch from a 16bit to a 32bit CPU in the early x86 days, of course performance
can rise. This article here takes a look at the environment today, and puts Windows XP 64bit in a variety of scenarios, both with “legacy” 32bit software and without, to see how performance stacks up. This time around, only the Athlon64 is used, though an EM64T P4 will be used soon. It goes briefly into describing how a 64bit OS runs 32bit software, and what the potential performance deficits are.
Some of the tests are very interesting, with 32bit outperforming 64bit. In fairness, however, a lot of it has to do with unoptimized code. In some areas such as 3d processing, 64bit Windows gained a considerable advantage. I'll expect to see similar results when the P4 is examined, because the EM64T implementation is almost exactly the same as the A64.