Originally Posted by NGUChris
AMD/ATI GFX cards are generally cheaper and support full audio over HDMI (nVidia may now have this feature I am not sure but ATI had it first)
Nvidia has been doing HDMI with full Audio since the 8800GT (it needed a cable plugged into the motherboard though). Now it's just like any HDMI device and does full audio and video with just the card plugged in and a cable.
For me, I'd do SLI simply because I"m an Nvidia fanboy. In the end, how much you notice it will mostly depend on the cards you are using, and the application you are running. One setup will scream in one setup and run like crap in another.