I'm not sure what you mean by letters, but you can activate cheats by going to your cell phone in the menu and calling numbers like #909.
Here's a list of Saint's Row 2 cheats for the PC, I'm pretty sure they're the same for Xbox 360.
http://www.cheatcc.com/PC/saintsrow2cheatscodes.HTML
in grand theft auto
X,L1,SQUARE, R1, O, DOWN, UP cheat codes exist in grand theft auto!
that's a mod.
go to cccheat
go to gtasanan.com
Hello
in the phone
no that's a dumb question
Cars
shift i
type in your keyboard all missions done in grand theft auto 12345678910
Yes