if you're looking for an excellent brand of screen protectors go with hori products
http://www.play-asia.com/paOS-13-71-b7-49-en-70-196x.html
if you're paranoid like me you can get the "cover everything" one which covers the outside shell, the buttons inside and both screens
http://www.play-asia.com/paOS-13-71-b7-49-en-70-1jbm.html
For games, most of the ones I enjoy have already been mentioned, only ones left would be
Advance Wars: Dual Strike (by far the best strategy rpg to grace the ds so far, and its sequel)
Brain Age 1+2 and Big Brain academy (all 3 can be grabbed for $20 each, but i'm sure you can get there cheaper)
Castlevania: Dawn of Sorrow or Portrait of Ruin (if you enjoyed metroid you'll love this)
Dragonquest heroes rocket slime (quirky platformer where you play as a slime, lasted me a good 22 hrs)
Elite Beat Agents (DDR+stylus = WIN)
Zelda Phantom Hourglass (self explanatory, direct followup to windwakero on the gc))
Pokemon Diamond or Pearl (if you're a pokemon fan)
Tetris DS (currently playing, its fun to play in short bursts)