The new PCI-E v2 cards will work on the DFI SLI-DR etc. I have an EVGA 8800GT KO running fine as said. There are rumours that future ones may not have backward compatibility - so in the months to come you'll have to check.
What is clear though is that whilst these cards may not need the full x16 bandwidth, they will not (according to the docs - I've not checked) run in an 8x slot.
So ... you can probably have an SLI rig with 2x 8800GTX each running happily in 8x slots ... but you CANNOT have 2 x 8800GTs because they will see they are in an 8x slot and error.
If you are looking at doing SLI on these boards (that only support 1 16x or 2 8x) you CANNOT use 2 x 8800.
I have one as mentioned, with an FX-60 and 2GB and I get just under 10,000 on 3DMark06 which seems like a really good result for an single card AMD rig. The only significantly faster rigs I've seen have been SLI ...
I think the later Intel based boards also make a difference but the FX-60 seems to be up there from the 3DMark06 point view.
I'd be interested to see what sort of 3DMark06 figures anyone with a GTS, GTX or Ultra have.