HD7850?
It is already a lot better than my current HD6850.
It depends on what games with what settings you use.
Example:
I played always with 1920x1200 on my Dell U2412M with my old HD6850 and Q6600,
Starcraft 2 + HoS (2x AA, all medium)
Skyrim (4x AA, all medium high)
Mass Effect 2+3 (maxed all available)
Tropico 4 (maxed all available)
Torchlight 2 (maxed all available)
I say ...go for HD7850, HD7870 or higher...just make sure you get something you can afford...