The 6950 is nearly twice as powerful as the 4890. At that resolution you will be great in all but the most demanding of games. I'm not that much of an ATI/AMD gpu kinda guy so I can't really help you much with the brands or overclocking though.
at that resolution 1gb is fine. go 2gb if u decide to go higher resolution. my 2gb 6950 flashed never gets below 50fps on bf2 and same on bf3 beta it was always 50fps+ on a 1080 monitor. honestly at that resolution even a hd6790 is good and save yourself some $.