a- It gets 2 DLC packs that's worth around 10 hours of gameplay, first one is in August. It's probably a new, small island with new side missions/ characters.
b- Live is much better. But taking into consideration all Live users have headsets, you're bound to hear some of the worst profanity ever. Nothing you haven't experienced in a Halo game though. PS3s don't come bundled with headsets, so mostly serious players/gamers are the ones to buy them, and they're mostly mature enough.
c- Achievements have no effect on the gameplay. It's stuff like medals ( some give free points on XBox Live ) for killing 100 pedestrians in 5 minutes, finishing the storyline missions in 15 hours etc. . Think about it as your e-penis on XBox Live.
d- Both versions are similar graphics wise, with only tiny unnoticeable differences.