I recently got a deal on a Zotac GTX 560 2gb card for $100. My primary video card is an MSI N560GTX-Ti Twin Frozr II 2GB-DDR5/OC. I knew going in that the two cards may not be compatible for SLI but figured I'd try anyway. Nope, I couldn't get them to work in SLI until I found this thread. I must say I am very pleased and impressed with the performance of adding a second GTX 560. I have both cards OC to 930 GPU with 2200 on the MC and I haven't dropped under 60fps on ultra everything at 1920x1080 and average in the 75-85fps range when playing BF3.
My only issue is with the test mode watermark. I've tried to get rid of it but unfortunately the only thing that works is method 1 (bcdedit /set TESTSIGNING OFF) but doing so requires me to reinstall the nvidia driver after I reboot as the display ends up at an undesired resolution and windows is unable to update the drivers. I can live with the watermark though considering what I get for it.