That's probably a 7 speed you've got there, but
any of 'em will work (and will work on 8 and 9 speed too); BTW when you refer to 21 speeds it's for the combination of front and rear...you'll probably need a Shimano if I remember correctly (a friend's Marlin at least is Shimano), I'm guessing sub-Deore or Deore gruppo. Your shifters need to match up with the derailleur in terms of how they pull the cable; SRAM uses a different actuation ratio. I doubt you'd notice the difference in performance if you went with a "better" derailleur than what model you've currently got, and it will definitely cost you more (a Deore will probably set you back 35-45 at most shops, LX a bit more than that, XT a bit more than that, XTR a lot more than that). If you want to "upgrade", save up yer money and go for a better bike...