Help with Graphics Card Please

[joddle]

I have several Dell 755s but the onboard graphics cards won't play some of the games my son has. On one game it specifies the following minimum spec for the graphics - 256 mb ATI Radeon 9600 or Nvida 6600 or superior.

 

I have looked for these cards but they seem only to come up occasionally on Ebay and sometimes with some sellers I would not really want to trust too much - I did manage to get one card two years ago but now we have another PC and so need another card.

 

I suspect there are many cards which beat the minimum spec given but I have no idea which ones could be suitable - so the question is - what other cards could I use? and how do I know if they are "superior" to the ones listed? I don't intend spending a fortune for the latest gizmo as my boy is only an occasional games player.