I only recently became aware that SLi and X-Fire only work in full screen mode. If you run anything in windowed or full screen windowed you are not using the SLi/X-Fire feature. Apologies if you know this already.I now sli 2 980s and I swear it looks/feels/works the same as just having 1... I must be doing it wrong.
There is an option in the catalyst suite to display a logo on screen if X-Fire is working. Maybe nVidia has something similar?
I went with AMD a long time ago, because cheaper. I've stayed with them for no reason. I would gladly give up my X-Fire 7970's if I could afford anything else.