Taken direct from nVidia (Pre 7900):
SLI works for graphics cards within the SAME GPU category. Here are the GPU categories again:
GeForce 7800 GTX 512
GeForce 7800 GTX
GeForce 7800 GT
GeForce 6800 Ultra
GeForce 6800 GT
GeForce 6800 GS
GeForce 6800 XT
GeForce 6800 LE
GeForce 6600 GT
GeForce 6600 LE
You must have two of these same GPUs in order to enable SLI.
And, because the 7900 uses a different chip to the 7800, you can't.
Originally Posted by adas
The cards have to have the same GPU, same amount of ram, and the same pcb. They used to need to have the same bios'es but newer drivers only require it be the same cards.
But to answer your question, nope, 7800GTX 256 will only work with anohter 7800GTX 256, and same for 7900s and 512mb versions
That's untrue, a 128MB 6600 can be paired with a 256MB 6600 with recent drivers,
(However, the 7800GTX 256MB CANNOT be paired with the 7800GTX 512MB, because the cores are different.)
Hope that helps