I haven't had too much experience with durons, but from reading these boards, and having my own duron 600 machine now, i'd have to say go with the 600. More people have had higher speeds with the 600 as compared to the 700's. My 600 does 1000 easy with 10 x 100, at 1.75v. Right now though, i've hit my sweet spot with 107fsb x 9 which is 963. It's 40 less, but i'm only using 1.625v and rock solid with whatever i throw at it. I've got a fop38, and it runs at 32c while idle. My case temp is 18c. The temp is not too far off, considering when i run it at 600, with default voltage of 1.5v, it gives me the correct temp of around 27, or 28. AGH! well that's just my two cents, go with the 600!
balael