also, once you hit level 8 your star ranking Glows.
BTW, I went for my fly the co-op achievement and got to level 6 while 90% of the time quitting out of matches before I got the money at the end. Had I stayed, I probably would be around level 9. So you should not try to "go for" this achievment, as earning all the other multiplayer achievements will net you pretty close to $5 million anyway. Then you can just play some games for fun to get the rest of your money. I personally enjoy cops and crooks the most.