To play at least CoD4 with medium settings, you'd also need at least -$80 cards from AMD. The Sempron will hold things back though, since CoD4 requires at least 2,6Ghz of dual cores. And the 2GB RAM if it indeed Dual Channel should help a lot, if it's Single Channel, things could get worse.
A 6670 will lighten things up by some degree, and with Sempron, it shouldn't tax your PSU too much. My best bet, you can play CoD at low-medium settings.
Here is a site where you can try things : http://www.systemrequirementslab.com/cyri/