Use R1 for firing. I come from the xbox 360 background and I started by R2 as triggers. The problem with this is that not all PS3 games have support for R2 as trigger. Another problem is that its a bit inconvinient to use the R2 buttons like triggers on the PS3.
Using the R1 for shooting and L1 for aiming is the best. Sensitivity depends upon the game, like killzone 2 required a very high sensitivity for me, but CoD required medium to lower sensitivity for me.
Depends upon the game, I usually have high XY axis sensitivity for my games including Uncharted 2.







