Well IMO opinion unless they lower the price on my next build I am seriously thinking of switching to Pentium. I am sad to say it as a long time AMD supporter but I may have to switch sides.
It also depends on how much you'll want to spend. IMO, it'll be a vast improvement over current AMD's. Prices are supposed to be VERY expensive when they're released, though. So, the question is... how long before the price comes down to a reasonable amount. If it were me, I'd go ahead and do a build now with a moderately priced Barton. Then in 6 months, or whenever, you can upgrade to an AMD 64. Then you can use your Barton as another cow for running FaD!
I agree with Ultima, the prices should come down for the current Athlons and mobos, since the Athlon64's require a new socket. I've read here and there that it will be released in late September. So, Im just going to hold off and see how the prices go and decide on an upgrade then....