It looks like your mobo: ASUS M4N78 PRO - motherboard - ATX - GeForce 8300 specs and Motherboard specifications - CNET Reviews
Supports nVidia SLI "NVIDIA Hybrid SLI Technology Support"
It has onboard nVidia graphics "NVIDIA GeForce 8300 Shared video memory (UMA)"
I dont know if Hybrid is real SLI.
But you might be able to use two nVidia cards in SLI for it I think.
If so,one can be for high end game processing and the other a lower less expensive card that has PhysX.