It it possible to SLI an 8800GT with a 260GT? If not, do the newest models support mixing models in SLI?
I was wrong, just done a bit more research and tried it here on a spare machine and some stock cards I have on the shelf.
If you try multiple cards from the same generation - 7x series, 8x series etc, it works at the speed of the slower device.
If you try multiple cards from different generations - a 7x series with a 8x series, once it worked but the second card was simply dead and displayed nothing, in a different configuration, it just won't display anything on either cards.
I can confirm that nothing is broken and both cards work fine, so you have nothing to loose by trying it, however based on what I have just seen, it will only work if you use two from the same generation.