I think it is worth it - I cannot compare an AMD 64 to the AMD XP 3000 but I have seen the 64-3200 against the 2600 and there is no comparison; I imagine the difference between 3000 and 64-3400 is probably about the same. It is worth getting; if you wait then you'll still be paying the same amount of money cause you'll want to 3600 or the 3800 instead.
It depends really on whether you're prepared to wait and whether you can get by for the moment with the 3000. Its probably better to wait though - the Intel 64s are coming out in 2 years!
but I don't think you'd be prepared to wait that long!