Originally posted by: Surutcra
you will not be able to use directx11 whatsoever with any graphics chip currently on the market as far as I know
You can use DirectX 11 on all DX10 and DX10.1 hardware, and also on various DX9 chips (The so-called 'downlevel' fuctionality).
However, there are some features that will only work on DX11 hardware, such as the tessellator option and the new SM5.0.
Other features, such as multithreaded rendering will work on all hardware, and the new compute shader will work on some DX10 and DX10.1 hardware (as far as I know, CS4.0 will work on all nVidia 8-series and higher, CS4.0 and C4.1 will work on Radeon 4000-series).
So to answer, DX11 will have some features that your GTX260M can use, and some that your GTX260M cannot use.
Regardless of whether you're using DX10 or DX11 though, Windows 7 will improve your 3D performance anyway, because it has a more efficient display driver stack than Vista. Windows 7 will also reduce memory usage, so you'll get more mileage out of the same hardware.