You can't SLI cards with different types or amounts of RAM (hence the GDDR3, GDDR5 comparison)
You are basically saying I am right by saying the 275 and the 295 are not the same card, but the same GPU and you can't SLI them.
What's with all the confusion. For SLI you need the same card model, period. Clocks don't matter.
What is with all this arguing about quad scaling. Quad doesn't scale, no game is parallel enough to make use of four GPUs without being held back by serialized CPU code, unless you run insane resolutions. Quad doesn't scale well at 2560x (4MP) and it doesn't scale at all at 1920x (2MP). It's for a niche market who have 3 monitors, best scaling would be at 7680x (12MP), but even with 5760x (7MP) you will have decent enough scaling to make the investment worth while.
It has nothing to do with CPU bottleneck. It's the programming that holds back the graphics and there is nothing you can do about it because all games have serial portions of code. To get these setups to scale, you have to increase the parallel workload portion, which is the resolution.
And stop saying that AMD has quad scaling working while NV doesn't. Nobody has ever tested NV scaling at eyefinity/surround resolutions yet, the highest tested was 2560x1600, and at the resolution no vendor has a quad setup that is a worthwhile investment.