There's also the matter of bothering to upgrade to a 64-bit OS in the first place. Assuming you have a reasonably expanded system, Windows 32-bit will see somewhere between 2GB and 4GB with PAE disabled after your hardware is mapped into place. Unless you're working with the some uber memory intensive applications, that's still quite a bit of memory.
The only thing I've upgraded in my box over the last three and a half years is the video card: Athlon 64 4000+, 1 GB RAM, 6800GT to 7900GT to 8800GTS. I've had my eye on the Core2 Duo and X48 and P45 chipsets, but I'll probably wait until early next year to upgrade. The Core2's are *fast*, but apart from playing a few new games, there's no compelling reason to upgrade.
Give it another year or so, and mainstream support for multicore 64-bit processors will be there. For now, the software (applications and drivers) is still bleeding edge, even if the hardware isn't.