You don't need new wheels, just slicker tires that will roll smoother on the road. There are some tires like that for mountain bikes. They don't have to be really narrow - 1.5 inches should be fine. The biggest difference there is to be noticed is that it won't be as hard to push slicker tires as it is knobbies. Beyond that, narrower probably won't make that much difference. But I sure wouldn't go to the great expense of buying new wheels to put those tires on. That would be ridiculous.