if you are maintaining inner wheel diameter, you are going to screw up the overall circumference which will mess up your speedo and odometer. However, IF you want to do it, you would need 195 (#less than 60) R 15's. The middle number is the ratio of the first number that is the sidewall height. So a 195/50R15 has 10% less sidewall than a 195/60R15 however it will have an overall circumference difference as well. So when going 60 mph on your real tires, you'd be going 65mph with the new ones.
I've heard that 205 55 r15's work qute well on the stock rims. Not looking at a smaller sidewall, but you are loking at wider. Also with a 55 series, there is less sidewall flex and more tread pattern touches the ground.