Cfd

Ditto on the non-matching treads......according to Sheldon Brown. That does make logical sense.
The slickest tire you dare use on the rear (where most of the weight lies) will considerably reduce your rolling resistance; while a treaded tire on the front will make for better handling.

Also, the width does matter: Best to try 37-42mm on the front and 35mm on the rear.
As to the standard advice for not exceeding the +/-2x inner rim width limitations, my LBS mechanic says that it doesn't really matter so long as you keep the tire pressure up as high as possible so that the tires won't tend to roll off in turns. The higher pressure will also enhance your handling capabilities.

I've used the Ritchey Speedmax Cross (32mm) with success on various surfaces except for serious mud; they're not exactly great on wet hardpack. Although I'm not too thrilled with their buzzing on dry pavement, I've have grown accustomed to it.