A tip for the kart racing, if you beat EVERY mission (any rank) with any 1 character you get an improved kart for that character with high stats making it easier to win.
777th Achievement: A Small Consolation-10G (Call of Duty Black Ops)
20000 Gamerscore: Smash TV-75G (Portal 2)
1000th Achievement: Licensed Properties-30G (Alan Wake)
30000 Gamerscore: A Knife to the Heart-50G (AC: Brotherhood)
GT: AtR ZacAttack