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I'd get a sleeping bag rated for slightly colder conditions than you think you'll encounter. I use a 20 degree bag in summer, and a zero degree (or better) bag as soon as the aspen leaves start turning color. You start to notice the cold as the temperature starts getting down to the bag's rating. The Rocky Mountains can get cold at night in any season. Sleep in the bag naked or as close as possible to it. This helps establish a warm layer in the inner insulation. If you wear clothes to bed, you won't warm up your bag and you might end up with annoying cold spots.
Avoid cotton clothes. I like wool, nylon, other synthetic stuff. Cotton gets cold when it gets slightly damp. Also, cotton socks cause blisters. Pitch them and get wool/synthetic hiking socks if you are going to do any walking. You can get outdoor-specific clothes at an REI-type store, or sometimes cheap fleece stuff from the discount stores. Use light layers. Don't scrimp on rain gear, though. Try to get seam-sealed waterproof, breathable stuff. Just plain plastic gets wet inside from perspiration.
Get more tent than you think you need, room-wise, one more person worth of space than is in your party. It also helps to have tarps and rope to make a rain shelter that is not your tent, a place for cooking and hanging out.
