970 seems to be causing prices to plummet, once they're readily available I would expect prices to drop more. Take a bit less now or maybe be looking at taking 300-320 in a month or two. 180 might be optimistic on the 760 too, even ebay prices on cards have started to sag.
I think a fair evaluation of a 780 right now is 250-300. And its only going down further. So yes, i would sell it for that price now and or risk for even lower prices in the future.
760 is about 150 max I would imagine.