upgrading GTX560 query

[mavers92]

Hi all, I was thinking to upgrade my (non TI) gtx560 1gb and was wondering if what I can afford is worth doing.
I use the TF mod and my screen resolution is a max of 1440x900@60hz
Most of my settings are on mdium to high, grass roads and shadows on and ik getting a rough 50-55 fps over water dropping to 25-30 over land.

I like the high settings as it does look wonderful when that high.

My options are HD6870 new or a used gtx660 / hd7850

Would the 2gb that the newer cards have be of any benefit as my screen resolution isn't that demanding (I don't think but not too sure if that)?
And is a hd6870 actually an improvement?

Don't mind about changing from nvidia to amd as I've used both, my last card was HD5770, also if I go amd I won't get that annoying "%1 isn't a valid window application" when trying to install new drivers.

I have looked at some other posts but the everybody had better specs than mine so the difference would be greater I think.
Other specs are--
I5 2500k @3.3 / 8gb ram / 120gb SSD / 750W psu /

Any helpful suggestion appreciated.