a quick fun game with linux version is FTL (faster than light). it lasts between 5 minutes and 90 minutes. depends how good (and lucky) you are
btw the GPU you have is preety good so many like Oblivion, Morrowind, Battlefiueld 2 will work on that. as well as half life 2 engine games (left for dead, portal...) that have linux ports etc.
edit: if you liked ET you might want to give World of Padman a try. i don't know how full it's servers are, but bots are preety good. graphics is nice and fun as well. and it's free. then there is also urban terror.
have a look at playdeb repositories....