Check out the stickied links in the back country forum for some reading info.
Shovel, Beacon, probe are the basics you should have. I think Wilderness Sports in Silverthorne rents beacons, but not many places do. You also need to know how to use them, so if this is your first time, I'd suggest putting an transmitting beacon in a pack and burying it in the snow, and then locating it. From 25 yards out you should be able to locate and get a probe strike on a buried beacon in under 3 minutes. Shouldn't take you long to get there, but this is something you don't want to learn under live fire.
GPS shouldn't really be needed unless you are prone to getting lost.