if you Czech Nymph, a longer rod is great. I fish a 9'9" 4 weight Cabelas rod for nymphing on the Provo and love it. If you are in Utah or SL County, you can go chat with Lance Egan who works at Cabelas. He is World Champion fly-fisherman and knows more about fly-fishing and tying than all of us on CB combined. He fishes a 10' rod on the Middle Provo.
That being said, I have a 9' 6 weight Redington Pursuit and a 8'4 weight Redington Path (the Pursuit became the Path) and like them both.
I prefer the 8' when there is a hatch on and I'm focusing on the surface. I like the 9' as an all-around rod and if I know I'm going to spend most of my time nymphing, the 9'9" lets me comfortably reach and control so much easier and I catch more fish.
Again, we all have our preferences, but if you are local, try to go talk to Lance.
Just plan on becoming addicted and having your mind wander to the river while you are in meetings, etc..