The brakes for you would be the Avid BB7s. They are the best mechanical disk brakes out there, and I am running them on the rear, while Juicy 7's up front.
There is really no use "upgrading" to ****ty disk brakes such as Tektro.. or even the Hayes MX2s for that manner, because they probably suck more ass than your rim brakes.
The BB7's are just a little bit more pricey than the other lower grade disc brakes but they offer hydraulic-like feel and power. You will notice the difference, especially if you are racing .You can pick BB7's up for like $150USD for F&R on eBay.
BB7's will last you quite a while as a mountain biker improving and they will last you until you really tackle difficult DH courses, in which case you should get 8" hydraulics.