Sink tip lines are great for fishing streamers and some nymphs in rivers. I still prefer a floating line for dead drifting most nymphs.
The right sink tip that you will use will depend on the water that you are going to fish. If it is a big river, a sink tip line with a 24 foot sinking section would be ideal. For smaller rivers, choose a sink tip with a 10 to 15 foot sinking section. You will also need to figure out the sink rate that will suit your needs. For shallow and slow, a ghost tip (intermediate tip) will do. For a moderate depth and flow, a type 3 tip would be the way to go. For fast and deep, go type 6.
Your alternative is to buy some sinking leaders. These loop onto a floating line. They will give you some versatitily without having to change lines.