Both cards are capable of playing most DX9/DX10 games very well at 1920 x 1200/1080 resolution but the 260, as well as the 4870, is about 10% faster than the the 5770. Neither card can handle Dx11 at this resolution, the 260 because it's not designed for DX11 and the 5770 because it doesn't have the bandwidth to handle the load. At lower resolutions, you'll have to weigh the performance drop versus DX11 support.
better for gaming, and resolution on my monitor is 1680x 1050, latter maybe buy 24" for higher res.
I dont think to cheange card for year or two. My mobo have 780i chipset, and is better to by GTX260, and latter , for about year take one more and SLi, or to buy now 5770 and in year or two by a new card?