i would go AM2. performance is pretty much the same. again, only major difference would be the jump from DDR to DDR2. which, for now, hasn't done much since the very loose timings undo any potential advantage from having faster ram (although 3-3-3-8 is decent, and not expensive). However, with AM2, you get upgradeability! i believe AMD does not plan on releasing the 65nm cores on 939, only on AM2. and on top of that with DDR2 getting better and better, it gives AM2 the advantage over 939. and since AM2 is pretty much the same price as 939, i would definitly go with AM2.
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius