Actually, it should work, but it will disable half the memory on the 640mb card. And you can flash a 7900GTO with the GTX bios or vise versa and run those cards in SLI. All that has to match is the GPU, the memory is not part of the equation.
Thanks guys, and welcome to the forum b1lk1- there's a forum special over at ncix right now on some of those 320mb versions- as low as $329 for the 500Mhz mode; great deal, but if it disables half of my ram on my existing card like you say, I'll hold out for a twin (and some decent SLI drivers).
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