Yesterday an identical Sparkle GTX 275 was delivered and with much anticipation I set-up SLI and ran 3DMark06. No gains at all over the single GTX 275 :-(
I then did everything you would think of: swapped the cards around; tested both separately; used a different sli bridge; moved the sli bridge to right hand connectors; used 2 sli bridges; uninstalled the forceware driver; downloaded the latest 197 driver.....
Also I should add that I have been running 2 x 8800 Ultras in SLI for ages with all the performance gains you would expect. This is so irritating. Tempted to just sell the 275s and buy a 295. Not sure if that will cope with running Crysis at 2650 x 1600 though :-(
When you install the drivers, does the driver recognize both cards and black out the screen once than under the nvidia configuration screen does it show that both cards are recognized and the option to turn SLI on or off? Just installing the driver does not automatically enable SLI even if both cards are recognized but the first thing is to determine that both cards are seen.