You will notice a difference going from knobbies to slicks, which is what I do it with my old mountain bikes. If you don't want to invest $80 in tires, then spend less. I have read good things about the $16 Forte tires, but I haven't used them myself.
Forté Gotham ST Road and Mountain Tires
You can always wear the knobbies out, then replace them with slicks, but they are worth converting.