Nvidia in Linux-land supports SLI in a limited fasion.
Thing is is that SLI is a silly gimmick in reality to get people to buy more video hardware then they need. It's kinda stupid and only works well on a limited number of games that support it in Windows.. the use for it in Linux is even less, which basicly means I can't think of any good reason to even have it. Of course other people will disagree with me.
Maybe you can trade your video card for another harddrive or more ram or a Hauppage WinPVR-xxx or Plextor PX-TV402U usb2 tv capture device and get Mythtv setup? I donno.
But since you have it.. it's a waste to ignore it.
See here:
http://www.phoronix.com/scan.php?page=article&item=326&num=1
Thing is is that the only Nvidia drivers that support it are 8xxx series drivers, which are currently in Beta. There have been drivers that have leaked to public that can utilize it. I don't know if Nvidia has actually released anything that they support, even in beta form. Nvidia has Linux-specific forums at their website and you can probably find the latest information about it there.
Good luck.