I agree on the clematis. You could probably plant one on each side, and they would go crazy! I have a narrow bed like that, and I had a lot of trouble figuring out what works best. I started with lots of perennials, but it just looked disorganized (but not in a good way at all). I think that you might want to do more plants of the same type. Maybe choose 3 different varieties, and plant enough of each type to fill in the bed. Boxwood are lovely, and are a nice evergreen. I also like euonymous - I have one that has green and yellow leaves, so it's very pretty. Be sure to look at the tags on the plants to see how wide they get. When we moved in, there were yews planted in the beds at my house, and the variety was wayyyy too large for that narrow space.
Another thing I found helpful was to go to a local garden store (not a Lowe's, but a place where they know what they're talking about) and ask their advice. OTOH, I do buy a lot of my shrubs from Lowe's because they sell quart size pots for about $7. If you don't mind being patient, those are a good option, too!
Just my 2 cents. Well, maybe that was a whole dime's worth of advice!

Have fun!