i think the less costly you can possibly get is GTX 580 but here price probably are different from where you live so ill tell you some more graphics card that should be fine
But anyway we need to know all your other pc component to be sure that
1)All your component are compatible with a new graphic card that you choose
2)You need to be sure too that your other pc component won't limit the graphic cards
example:
if a game require 5gb of ram and a g GTX 660 if you have 1 gb of ram even if you buy...
i think the less costly you can possibly get is GTX 580 but here price probably are different from where you live so ill tell you some more graphics card that should be fine
But anyway we need to know all your other pc component to be sure that
1)All your component are compatible with a new graphic card that you choose
2)You need to be sure too that your other pc component won't limit the graphic cards
example:
if a game require 5gb of ram and a g GTX 660 if you have 1 gb of ram even if you buy a graphic card that is more powerful then a GTX 660 the game won't work so you need to check that your other component are enough to support the game and the new graphic card