I was recently in the very same position as you, to upgrade to a 7850 or a GTX 660. Then I changed my mind completely and ordered a TrackIR 5 and a Saitek X-52 instead since I figured those two would make a better improvement for my simming needs than any graphics card BUT since I did a lot of research about the two cards I'll give you my thoughts on the matter.
The answer is easy, GTX 660 if you can afford it. If you can't then the 7850 is a good budget alternative. The 7870 doesn't have as good €/fps ratio as the 660 I'd say so I would rule it out completely. The GTX 660 beats the 7850 in almost everything and gives the 7870 a real good run for it's money. If I had 200€ to put on a new graphics card it would easily be the GTX 660.
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