If I don't have to pay a price premium than the 64 bit would be fine. But since nothing I use will take advantage of the 64 bit processor the price would have to be comperable to a 3.0 GHz P4. BTW, what make them better than a 32-bit processor. Seems to me if you run Windows XP or other 32-bit OSes you are just going to force the chip into 32-bit mode.
It's not just a 64-bit processor. It's also has the memory controller on processor. That means that memory access is a lot faster. If you check on most hardware review sites, they show the A64 whooping the tar out of Athlon XP (and even some Pentium 4, depending on the flavor and the benchmark).
If you really DON'T want a 64-bit processor for whatever reason, then I would suggest examining the Athlon Sempron 3100+. It's a 32-bit version of the A64 and is slightly cheaper than the A64 2800+.
The only reason at this point to stay with the Athlon XP is if you want to save money (which is a good reason). But in terms of upgrade path, the Athlon XP is dead.