i think that is not possible. for nvidia card you need the same model to do SLI. though you can mix the vendor brand or card with different amount of VRAM
Umm no. SLI is not as forgiving as crossfire. Even though both cards are based on the G71, they have a different number of shaders and ROPs. Perhaps you could accomplish this with hacked drivers, but that's a whole other set of issues really .