Hum, you do know that Athlon64 work perfectly & natively with all existing 32-bit hardware, OS, drivers, software, games, etc. Not only that, but when running in 32-bit, it runs faster then any other 32-bit CPU currently available (Pentium-III, Pentium-IV, AthlonXP, Duron, etc.).
If you also wish to have the latest in hardware such as PCI-e, it's the only way to go. You also get a NX-bit capable, cool'n'quiet, SSE2/SSE3 running chip. There is simply no reason at all to get an AthlonXP at this point in time.