The most important spec we need is your PSU make/model. We can't really recommend a card until we know the PSU can handle it. And whether you have a full size case or a slim line.
The little Sandy Bridge dual core can work well with most mid range cards. Faster cards will work too, of course. But you will begin to suffer some CPU bottle neck in some games. And multilayer gaming on a busy server will be rough on the dual core. That will drag down its ability to keep up with the card too. I would feel comfortable with an HD 7850/GTX 650ti or less playing single player games. You might still experience a tad of bottleneck in some games, but not enough to be a deal breaker.
If you see a CPU upgrade in your future, you could buy...