Replying to Vid. cards

So i got the orange box, and Half life episode 2, portal, and team fortress 2 didnt work. so i figure my computer is fairly old so i need a new video card that can support these games.

My qusetion is can I install a new video card, like an nvidia 8800, in a computer that is 5 years old? or do I need to get all new software?

It already has an nvidia 440 mx in it, which has 250 mb ram. I have all of the memory I need, i think I need a card that can support directx 9.