The reason for the foam underlayment is to smooth out the small variations in the floor level allowing for better installation and to allow the floor to expand and contract smoothly (this is why its called a floating floor). Make sure to check the stuff you get before you buy and see if there is alot of chaff or debris in the lock channels, if there is it is a PITA to install and your better off paying a slightly higher price for simpler installation. Im not sure if the stuff has foam padding already if you need underlayment, but it cant hurt and will provide a solid vapor barrier, and it is cheap enough by the roll, you absolutely do not need the more expensive stuff, the stuff we use when doing it is about an 1/8 of an inch thick and will cover quite a bit of floor and has adhesive along one side and we buy it on sale from Menard's, Lowes, or HD. And do source different places, there can be quite different prices for different bits of material. One job we did we got the flooring one place, the underlayment another, and trim off of craig's list and cut transtion strips from the trim to fit the floor.