If you plan to stay on 1080p even with a 120hz monitor, i'd say go with a single 280x. You see somewhere around ~50% more performance with a 280x than a 7850. If you plan to fo multi-monitor though, i'd have to suggest 290, as that is a sweet spot for what performance it offers at that resolution.

 

R9 290 is 25% faster than R9 280X and is the best price perf card . period. wait for an asus r9 290 dcii top or his r9 290 iceq x2 turbo. or grab a ref card and do a accelero xtreme iii / gelid icy vision cooler mod.

btw the R9 290 more than doubles the perf over HD 7850. a single R9 290 overclocked will max any game at 1080p and run very smoothly.

 

As someone who went from an overclocked 7970ghz edition (probably 10-15% quicker then a 280x) to a 290x, I can't argue against the $400 290 here. My 290x can max anything at 1080p w/ AA, but my 7970ge definitely started to struggle on some newer titles such as BF4 and Crysis 3.

 

If you're planning to get a 120Hz monitor then you will want to upgrade to a Radeon R9 290.

Your 7850 won't be able to push enough frames to be able to utilize the benefits of having a 120Hz monitor.

 

loos like the 290 is the best option, just gotta wait for the after market coolers