Right now, i'm running 42-18 with a 26X2" rear tire.
Maxes out at ~22-23 miles an hour, according to the speed radar on my commute, though i could probably spin a little faster if i had clipless pedals rather than bmx platforms.
If real winter ever arrives in boston, i'll drop that down to 42-20 to cope with snow plowing and heavy studded tires.
Off road, i've gone as low as 32X18, which is pretty great for any type of hilly/bumpy riding.