dcrz-

The best advise would be to go to a flyshop and practice cast a few rods to see what fits your style. I used to always fish a 6wgt because of wind, bug size, etc. Now I almost exclusively fish my 5 piece 4wgt Sage. I still live here in Colorado and, believe me, it gets windy up in the canyons just like Montana (I've fished there to). The advise for the 5 wgt is sound since you may get the urge to hit the Yakima after you get completely hooked on flyfishing in Montana. It blows all summer there. Once you become proficient at casting, you could go with a 4wgt just fine. They are more fun since they are lighter weight and make average fish (Yakima size) feel large. They also protect delicate tippets better than 5s or 6s. Don't even consider a 10'. The standard length is a 9' and every major manufacturer makes them in this length. Regardless of what weight you go with, buy it in 9'.
Post your question on www.flyshop.com and see what kind of responses you get. Its fun to get those guys riled up on topics such as this.