Life is a bit of a pain at the moment, with Vista around the corner (allegedly) and another performance leap in hardware to go with it.
A pragmatic view might suggest that whilst you can currently buy a 64 bit machine that is going to be Vista ready and oozing performance, you are certainly going to pay for it. I am just organising a new PC for friends and they have the same problem as you, in wondering which way to leap.
My own feelings are that you can get a good little cheapo now, which will leave your existing machine for dead and then buy again in about twelve months, when the new performance levels are becoming more mainstream and you are not paying a hefty premium for it.
What is more, you will get a faster machine then and in all probability, it will be so much cheaper as well, that you will almost pay nothing for your "pragmatic" PC now.
a 3,0Ghz P4 based system now, (Or equivalent Athlon - I would) can be picked up for peanuts and will still be eminently usable in 18 months as a second level machine - maybe a dedicated Ppruning platform.
I know that a vaiation of this argument can be applied almost any time with PCs but it is only marketing hype about 64 bit that is selling anything at the moment, as people wait for Microsofts next bag of bolts..Just think about it.
Conan