From the Geforce SLI FAQ website: http://www.geforce.com/hardware/technology/sli/faq
Can I mix and match graphics cards from different manufacturers?
Using 180 or later graphics drivers, NVIDIA graphics cards from different manufacturers can be used together in an SLI configuration. For example, a GeForce XXXGT from manufacturer ABC can be matched with a GeForce XXXGT from manufacturer XYZ.
Can I mix and match graphics cards is one of them is overclocked by the manufacturer?
Yes. A GeForce XXXX GTX that is overclocked can be mixed with a standard clocked GeForce XXXX GTX.