Premiere Pro can only use ONE video card, so your best option is to get a new video card, something like a GTX 560 Ti or better and pull the GT 9800 out.
What Harm stated assumes that you'd be editing video using Premiere Pro CS5.x.
On the other hand, it appears that you'd be simply playing back video that already has had slow-motion effects. In this case, playback is entirely CPU dependent, and a second graphics card offers practically no improvement at all whatsoever in the playback performance. Thus, it is likely that your system and CPU are too old and too slow for smooth playback of video, especially high definition video.
By the way, you did not specify which CPU and how much RAM you have.