Our house has central heat and AC except for the attic..which happens to my bedroom! It gets blazing hot up there! Even if it's only 80 outside, you can expect the temp up there is over 100. I've got my AC cranked on 8 (my window AC goes from 1 to 10) and even then it can get kinda stuffy.
As for the downstairs, we've got it set on 72 and it's just perfect! The cats love it, but yet they always want to go to the basement since it's even cooler down there. Plus, it's got a concrete floor and it's almost chilly to step on to it...but those crazy cats love it!