You mean a GTX 980, which is currently not released, or the ancient 9800 GTX?
Either way, not really worth it.
A single GTX 750 Ti would outperform two 9800 GTXs in SLI.
And a single GTX 980 would be so much stronger than a GTX 750 Ti that it wouldn't be worth using as a PhysX card, let alone two GTX 980s.