Insulation, Radiant and Vapor Barriers - R-value benefits?

I'm getting ready to reinsulate my attic after some remodel work. We had the old insulation sucked out (blown fiberglass), for the remodel, and because the previous owner allowed cats to live in the attic (giant litterbox). I've decide to insulate with cellulose (Cocoon) insulation but I'm not sure what R-value to go to with. I live in Bozeman, Mt. All indications are that I should go to R-49, but I've been told by several people that benefits from R-values higher than R-40 are negligable. Does any one know if this is true, is it worth the extra cost to go to R-49 or even higher? :confused:

That is still up in the air about the R. Looks like a R 38 and R 40 work out the best so far and works the best . So Id go for it. Cellulose for sure. Do you have any V/B down on the ceiling???? If not Id put a R13 with paper side down to the room side or ceiling and put the cellulose over it all for the R-40. Dont forget the foam rafter vents so the air can get up over the insulation from the over hang.

ED ;)

:) Thanks Ed, I do have venting taken care of. I also have some fiberglass bats installed over the bathrooms since I plan to do some work in there later on. I got started on the cellulose today, man does that make a mess! I've decide to go to R-49 since we spend a lot of winter nights up here below zero. Thanks for the advice. :D