A window fan could work, but if you have a peaked ceiling that is where the hot air is going to concentrate. You will probably need interior fans to move that air in the upstairs combined with a window fan to exhaust the hot air. Dont forget an open window.

Perhaps one of those 2 fan window units? One side blows in fresh air while the other side exhausts.

Thank you for your reply, busboy! Definitely, the fact that my 2 nd floor is immediately under my roof is the main cause of it being sooooo hot. I've tried the little two way fans, but they weren't terribly effective. I was hoping someone would know of a stronger fan option. I would love one of the old-time whole house fans that fit into a window.