No point getting a 64-bit processor unless you're going to use a 64-bit operating system. (though Windows will still work on the AMD-64, albeit perhaps fractionally slower than on an equivalent 32-bit AMD CPU). I don't know what the "sweet spot" is now for the Athlon 32-bit XP range.
Longhorn (M$) is a l-o-o-o-o-n-g was off and will be horribly expensive and full of bugs. SuSE Linux 9.0 for the AMD-64 is here, stable and working (but I's wait for 9.1 which will be out in a couple of weeks).
But if, as I suspect, you want to go the M$ route you might as well get the P4 (although IMHO the Athon XPs are the better chip).