If you are anywhere near Peck, Idaho, Poppy
at the Red Shed Fly Shop
is a valuable source. He can get you dialed in for your Spey needs.
Here are a couple of articles to read.
Understanding Spey Lines 2010
by Simon Gawesworth
Skagit Fly Lines
by Eoin Fargrave
Your line choice is going to depend on the style you are going to fish. Watch the following video, ABC's of Spey Casting with Steve Rajeff, that was produced by Leland Fly Fishing Outfitters in San Francisco. Steve Rajeff describes the three major styles of Spey casting.
Most of the year, I fish Skagit style. I swing all sizes of flies deep for trout, steelhead, and American Shad. I also find it the easiest style to fish.
During the late summer into fall, I fish a hybrid style of fishing between traditional and Scandinavian using short headed lines (head around 53'). Do not get these lines mixed up with the shorter shooting head styles (Scandi and Skagit). When I use this style, I am either swinging traditional style steelhead flies or skating surface flies.
I have plenty for you to get started with. If you want to chat specifics, go ahead and continue on the thread. I'll follow up.