You can use the gt220 as a physx card and the 6870 as your main gpu card. But your mobo needs to have 2 pci-e slots. And sometimes it is not worth it, because it might bring down your performance. So best thing to do is to find out whether you can run those two cards together in your rig, and then test it. Benchmark only with gt220, and only with 6870 and then one combined. That will give you the answer if it is worth it. You already have a gt220, so you can try it, but I would not go out and buy an old gt 220 just to do that.