You may gain a bit but not a whole lot unless you have some serious knobbies on there now. If you want to keep some offroading capabilities, I'd go with these:
http://www.nashbar.com/bikes/Product...2_10000_202470
I've used them (or the Performance version of them) on every MTB I or my family have owned for at least a decade. They ride nice on pavement but still do OK on dirt and gravel.

If you are going to stick to the streets, go with a narrower, higher pressure slick tire. Something like these:
http://www.nashbar.com/bikes/Product...1_10000_202472
if your rims are narrow enough.