Everyone's heard a bunch of stories on what you need for SLI. Well, here are the requirements, DIRECT from the manufacturer's!
You need to have several things, otherwise, it won't work at all!1) The cards MUST be of the same type (eg. 2 6800gt's., 2 6600gt's)
2) The cards MUST be from the same manufacturer (eg. 2 XFX, 2 EVGA)
3) The cards DO NOT have to be the same revision.
4) The cards SHOULD have the same bios. Compatibility issues may arise if the bioses are not the same.
5) The cards NEED to be the same specifications on the box (eg. either 2 VIVO, or none VIVO)
And then there are your basics that you will need for your new uber-rig /smile.gif' border='0' style='vertical-align:middle' alt='smile.gif' />1) A power supply from a quality manufacturer (e.g. Antec, E-Power, Enermax, etc.) that has at least 350W and >20A on the +12V line.
Note: You don't need the amount that Nvidia requires, such as the 600 or so watts for a dual 6800 Ultra. They just say that for the people who don't have quality psu's.
If you do experience problems, first check your power supply. Make sure about the amps, and that that it is from a quality manufacturer. Cheap power supplies = dirty power to your computer
Second, if its not working, it may be the video bios. If they're not the same, the cards are getting different instructions, and with that, they wont be able to communicate to each other.
Unless you are experienced with flashing video bioses, contact the card manufacturer, and they will help you locate and update the bios, or, you can ask to rma both cards so that you dont have to risk messing anything up. (That's what I'd do)
If its not the bios, and you really can't figure it out, you probably should rma the cards, and that way you will have a matched pair direct from the manufacturer.
Remember, many card manufacturer's have toll-free tech support 24/7, so you can always call them. If the tech support is not toll-free, cheap phone cards work great...that's what I did /smile.gif' border='0' style='vertical-align:middle' alt='smile.gif' />.
So there you have it. The truth about what SLI needs regarding video cards.