I agree with the others who have said to pass on the 50-200 for the eventual 55-300, primarily because that extra 100 mm really does make a difference if you're trying to capture sports action from the sidelines.
One thing I can say in favor of the 50-200, though, is it does autofocus pretty fast. I've never used the 55-300 so I can't say if it is slower or not, but I wouldn't be surprised if it was a little slower given the large zoom range it covers.
The 50-200 is useable for outdoor sports, but you will probably have to crop often like I did here (using the K-7):
This was during a scrimmage where I was allowed on the field. I still would have preferred to have the reach of the 55-300 mm.
One note on technique -- I've found that the K-x isn't necessarily the greatest camera for panning with the action and machine gunning your shots hoping for keepers (a.k.a. Spraying and Praying). I've had a lot better luck with the k-x trying to anticipate action, even focusing on the ground where I think the players will be rather than on them directly, but this can be challenging and I haven't mastered it yet.
Here's one where I focused on the track with a wide angle and let the runner run into my shot. If only i had pointed my camera a little bit lower I could have avoided cutting his foot off!
(using the K-x).
I thought these tips might help a little bit. Enjoy your K-x!