Thanks to all the people that responded to this thread. I got a better picture of the different options available for me. The problem I face is that I've got a firm budget for this system, and can't go two or three hundred bucks over- which I would definitely be tempted to do for the benefit of a 939 system.
The way it looks right now, if I go with an XP-M based system, there are some secondary costs I could potentially cut, such as RAM (still trying to understand exactly what I could get away with in such a system). With a 939, it seems that I would be obligated to pick up TCCD, which is going to be great RAM, but is also top of the line, and there's the associated price increase. Being that the 939's are multiplier locked, and I'd be looking at a 3000+, I'd have to pony up for the chance at hitting a high FSB. I know that regardless of what system you're looking at, you're going to want to push the FSB to the max, irregardless- but it'd be nice to have a bit of a safety net.
There was the suggestion that I just try to downgrade on the computer and MB, and try and keep everything else suited for a A64 upgrade in the near future. There are a couple of problems I see with this:
First, wouldn't I need a super-powered PSU, like the Fortron 530 or the OCZ with the 939? I still would need a good PSU, but could maybe get away with something a little less powerful, and possibly less expensive, with an XP system? Those are more rhetorical questions, in a way, as I'm sure these issues have been beat to death, and I'm more than willing to go back and look for some of the already-existing answers to these questions.
If I go with the expensive RAM, but don't intend to upgrade the CPU/MB for atleast a year...it starts to become a question of whether I'd be better off to get something more value-oriented now, and then upgrade the RAM with the CPU/MB.