There are dual-cores which are factory-underclocked and they consume less than that Sempron. The Sempron is going to be faster at single-threaded tasks but overall the dual-core will perform better and require less cooling.
pointed out, the Sempron 140 is just a normal Athlon II X2 with the second core disabled. It's dirt cheap (or was when I got mine anyway) and you've got a chance of unlocking the second core.
A factory-underclocked dual core proper will be more expensive. So it all depends on if one really need that second core, if one can be bothered to underclock manually and how much money one like to spend.
I also would like to point out that while it's possible to cool things passively, if you can add just one slow 120mm case fan, spinning at 500-600 RPM or something you can cool a really quite powerful system. I'm pointing it out since the Sempron 3000+ system mb2
is using now actually had one fan according to the link in his signature. And with a mechanical HDD the system already has moving parts.. mb2
, how much CPU power do you need? For my disk server a while back I went from my underclocked Sempron 140 system (38W AC idle, one HDD spinning) to a less power hungry Atom system (D945GSEJT) but now I'm going Sandy Bridge because the Atom N270 is painfully slow..