For the most part I love my card, but it sometimes has issues running ultra on my 1080p screen (40in). on far cry 3 I'd hit about 36-40 frames on ultra. I'm just concerned that my card may have more issues as time goes on and new better looking games are released.
If you look at the benchmarks then the high-end GPUs also struggle. http://www.tomshardware.com/reviews/far-cry-3-performance-benchmark,3379-6.html
MY advice is to just turn off MSAA and leave everything else on ultra.
My old GTX 660 played it at an average 50 FPS without MSAA at ultra settings, with FXAA and at 1080p.http://www.youtube.com/watch?v=BRnKfQaVVwM
Your Radeon 7870 is a little faster than the GTX 660, so it should fare better in Far Cry 3.