are there any games that are $12.50 or less on steam that will provide hours of enjoyment on a low end pc?

im trying to find a game that will be pleasant to play for hours on end. i have a low end pc (6gb ddr3 ram but integrated ati radeon 4200 graphic card). i enjoy rpgs shooters and strategy games, but hate horror and puzzle (besides portal).

i now this may seem like a generic list but i dont know what to put down

edit: i already have terreria