The HD 5850 will play games at that rez well, like you said you will get 20 to 30 % more performance out of a HD 5870.
Its really your call, i have a HD 5850 and i am very pleased with my FPS in games. Even crysis i can do min 30 fps on very high no AA
IMO the HD 5850 will still be a good card in 2 years, you might have to start turning down the setting then. or buy a new GPU.
Everyone's different tho, i sell my PC every 3 years and upgrade the whole rig.