Ya don't worry about upgrading the rear derailleur until it actually breaks. RDs are a money pit - performance difference between middle of the road and high end is negligible and often times the high end piece is twice the price. But, if you wanna add "bling" that's the first place most people look ;-)

The best upgrade you can make to any bike starts and ends with the suspension (fork and shock) and the wheelset. This is where you'll notice the biggest gain in performance, and the biggest dent in your wallet! But, 'specially with a 29er, a good set of wheels will make all the difference in the world!

Welcome to the addiction...