Might be more aggressive than you want, but for a true dirt tire I would recommend Kenda Trackmasters.
+ 1 on the Kenda Trakmaster IIs!
These DOT knobbies are much more versatile than one might expect; work generally o.k. on pavement, true traction off-road, and reasonable mileage.
One caution: Beware severe lateral changes on hard surfaces; very little front tire contact patch area might precipitate a wash-out (don't ask me how I know this!).
I have two wheel sets; one shod with Trakmaster IIs; one currently with Kenda K270s. Have a set of new Continental Trail Attacks standing by; truly excellent tread, but more oriented toward the slab (where they're outstanding) than the dirt, seem to me.