For the same price, you could get a single core that was much faster than a Dual Core, and would perform better. But when Dual-Core games come out, you'll be stuck in the dark.
Here's one thing you can do. Get an Opteron 165, its a 1.8GHz Dual Core processor, exactly the same as an Athlon 64. However, they overclock much better than A64s and you can get the 1.8GHz Dual Core to 2.6GHz or higher 80% of the time. Thats as good as any single-core processor out there. So you get a Dual Core with single core power. Ofcourse its more expensive ($325) than a single core that would do the same thing ($170).
But then...you're stuck when the time for Dual Core games comes. Or you could upgrade your processor when those games come out, provided 939 is still around since AM2 is taking over. But you end up spending $170+$325 for processors, so thats kind of weird.