If it is the 192 shader GTX 260 it's just a little bit faster. But if it is the 216 shader GTX 260 than it is significantly faster (READ: faster than a Radeon 4870). If it is the 55nm 216 shader GTX 260 than it is definitely better as the 55nm GT200b chip overclocks pretty well making it as fast or in some cases faster than the GTX 280.
Most games use DX9, while the games that do use DX11 usually just add tessellation and a few DOF and lighting effects with DX10 still providing very good looking graphics (take a look at Crysis Warhead).