Athlon 64's vs. Athlon XP vs. Pentium 4

I currently have a Athlon XP 1.8 Ghz system. I'm very happy with it
except the motherboard is not fried. I wasn't planning to upgrade to
anything better for at least a year or two but maybe now is the time.
My first reaction is to go 64 bit to the Athlon 64 but yet I know I
won't use a 64 bit OS for some time and I know the ones to come out
(64 bit XP in 2004 especially and Longhorn) may have less features
than the regular XP so I hate to make the downgrade so am very
concerned about 32 bit performance but the fact of having 64 bit when
the time comes is nice. My concerns:
For the Athlon 64 if I go with that should I look at the regular
Athlon 64 or the Athlon FX? Is there a difference in 32 bit
performance among them? I know there is a big price difference?
As for the others, should I look at one of the faster XP's, maybe one
with the Barton core?
As well, how does the Pentium 4 compare pricewise and performance
wise?
As well, I'd appreciate any motherboard suggestions. I have used Asus
mostly and have been very happy with it.