Just go with whatever sounds cooler, lol.
Your Dad is right; If you want to fully use the processor, then you'll need a 64-bit operating system. But right now, Windows XP 64-bit is pretty crappy. Add that to how crappy windows usually is and you have why so few people use it. Besides, half the hardware doesn't even have 64-bit drivers yet so...it won't run.
The 64-bit processors will be fully utilized when REAL 64-bit OSes come out like Windows Vista/Longhorn. When buying a 64-bit OS right now, think of it as future proofing your PC since a 32-bit processor won't be able to run 64-bit apps.
Besides...all the high-end chips nowadays are 64bit. Even the Semprons (budget) and Turions (mobile) went to 64 bit.