In SLI you plug your monitor into only one of the video cards.
As for the 7950 GX2, I would think long and hard before buying that card. It's two cards built into one, with a GPU on each card. It behaves like SLI, except that you don't have to bridge the cards and only one PCI-E slot is used.
I have and still have many problems with my 7950 GX2 and it currently will not operate. First problems involved run/stop errors and computer rebooting (couldn't run anything with it); now I am experiencing artifacting and the computer freezes up. I have tried the 91.31 drivers (run/stop errors/rebooting), then I tried the 91.33 drivers and card ran find for a month (then artifacting started), then last nite I tried the 91.37 drivers and artifacting started about 5 minutes after computer booting. Temps never got above 59 degrees celcius prior to artifacting, so it isn't a heat issue.