One odd thing: Semprons appear to have a higher performance-per-watt than Athlon64s; with modern processes, large cache can apparently suck a lot of power vs. the execution units. From various benchmarks, I get the impression a Socket 939 Sempron 3400+ is at least on par with the low (3000+?) end of the A64 range; remember, the Sempron fake-clock numbers are supposed to compare to a Duron, while the Athlon fake-clock numbers compare to previous Athlons. They also apparently have a nice low consumption at idle — something like 9W for some particular model, though the A64s are only a little further into the double-digits — which was an efficiency concern for me replacing office desktops that spend a lot of time doing nothing.

If you want something dual-core, there’s a thermal sweet spot somewhere around the 3800+ x2 and, I think, maybe the 4200+ or one particular lower-numbered 4n00+.
(Using http://www.hothardware.com/viewarticle.aspx?articleid=710&cid=1 for a quick reference here.)

If you absolutely don’t give a shake about performance, apparently the new Geodes with the funny part numbers pop into Socket A. Of course, then your northbridge might have a louder fan than the CPU. No idea how this compares price or performance-wise vs. going EPIA.