i'd say keep it, the very very mild jump to the 660 (like what.. 5% ?) is hardly worth bothering for.. now IF someone would give ya $130-150 for your 7800 then yeah I'd jump all over the 7950, then coming out to $130ish.. but if you dont plan on selling the 7850 to recoup any coin, keep it.
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Fermi Paradox*: "The apparent size and age of Fermi die suggests that many technologically advanced GPUs ought to exist.
However, this hypothesis seems inconsistent with the lack of observational evidence to support it."