Welcome to the joys of PC-buying ... you go for top of the range white-hot vastly expensive - and then 10 minutes later something even better comes along. Gnash!
Longhorn isn't going to be around until later next year, by which time your now-new PC will probably be pretty middle-of-the-road. But if you check at the assumed specs for 64-bit Longhorn it might well be worth going for the 64-bit processor now.
FWIW, I've got a 64-bit processor, but I'm not going to put 64-bit XP on it because the upgrade path is a bit of nightmare and the 64-bit drivers are apparently all over the place. And there's not much 64-bit software either. But I will probably go for 64-bit Longhorn when it comes as I assume by then the drivers will have been sorted and software will have come along too.