pokagbamo :
http://www.tomshardware.com/reviews/radeon-r9-280x-r9-270x-r7-260x,3635-9.html
do not overclock the first, it will be enough too
My biggest problem was that you were not stated that you are a beginner, nor did you followed the guide of choosing GPU, so we did not had enough information about you, or the system.
That is the reason why toms and me ask for phrase and write in a nicely laid out form.
If you would spend 5 minutes on the article about the layout and phrasing, we could understand the question better, and we can give exact answer.
There are several categories in graphics card.
usually= 270x ( that is a rebranded HD 7870) has nearly equal power to a GTX 760= they are desinged for 1080p Ultra resolution gaming, and most of the time, they can give playable FPS in games at these settings
280x (yes, that is the HD 7970) is nearly equal to GTX 770. These are designed to play games at 1440 in ULTRA details, so they are more powerful than the tier below, and they can play games 1080p faste ( around %20)
Take a look around at the specs of the 280x, maybe there will be a short version, that will fit your case. May I ask what type of case do you own?
For price, GTX 760 would be cheaper. But for performance/price, 7970/280x would be a better choice, and more future proof.
Toms hardware does benchmarking with loads of games. Please, if you can, use the benchmarks above. Only take a look at gaming benchmarks ( if you will do only gaming), because the synthetic and productivity programs are not reflect the gaming performance.