GTX 760 is ideal for 900p
R9 270x is littel bit less powerful than GTX 760, also ideal for 900p
Note: Both GTX 760 and R9 270x also works fine in 1080p in "Ultra Settings" with >40 fps in most latest games(you will get <60 fps in "High Settings" or "Medium Settings" with both cards)
R9 280x is ideal for 1080p( you can go "Ultra in Settings" with most of latest games <60 fps)
If you consider playing next gen games like Watch Dogs,Battlefield 4,Thief etc., then consider getting R9 280x.
Get the SAPPHIRE or ASUS version graphic card, Asus has high base clock(Both are good).
I suggest you to get ASUS GRAPHICS CARD R9 280X DC2 TOP 3GB DDR5
ASUS GRAPHICS CARD GTX 760 2GB DDR5 DC II OC EDITION...