It may also help to disable the secondary monitor while playing games too. But, as Clown said, those cards are not for performance. They're basically external cards, that performance at the same level as integrated, and only meant to give basic functionality to computer with no integrated video. I would suggest returning the cards, if you can. Then, get at least an ATi Radeon HD 4670, or nVidia 9600GSO. You could probably go with a 4650, I had one, and could almost max out Modern Warfare 2 with it.
Those cards will run you right around $60, but well worth it, in my HONEST opinion. The 4670 is closer to performance with an 8800GTS than, a 9600GTS, but the 8800 cost double what the 9600 does. The 9600 is also faster than the 8800.
Graphics Performance List