I'd go ATI 5770. Performance is really neck & neck with the 4870 all in all.
Yes it will suck for DX11 performance at high resolutions. But you don't have to use DX11.
Games will allow you to designate DX10, at which point the card should perform like the ATI 4870 anyhow.
@ 1680x1050 resolution the ATI 5770 is the way to go.
@ 1920x1080 (or higher) I'd go ATI 5850 if you can afford it.
@ DX11 Gaming - ATI 5850 or ATI 5870 are in order.