The GTX 260 (216 or not) is a cut down version of the GTX 280.
It has less stream processors, TMUs, ROPs, VRAM and memory bandwidth. The 260 you're looking at has higher clock speeds than the 280 because it is a factory overclocked edition.
A 280 isn't really worth it for 1680x1050, so it would be best to go with a regular 260 and overclock it yourself.
I'd actually go with a HD 4870
as they are quite a bit cheaper than the 260s ATM and perform pretty much the same.