Well, you don't 'have' to. You will see a speed difference regardless. But, in order to reap the real benefits you'll need an OS that can run 64bit style
That will get you halfway there anyway. Won't be 100% until there are apps optimized for 64bit. The cool thing about Linux is, it CAN be optimized for 64bit, while still remaining 32bit compatible. Windows -at this time and in the the forseeable future- cannot.
I am holding off till the middle of next year to start building my 64bit system. By that time, the FX51 chips (the only way to go IMHO) will become more affordable and Linux will be much further along with support for it. I want to feel that 2.2ghz FSB!!!