DS2X = 2x width of current resolution
DS4X = 2x width and 2x height of current resolution
DS6X which is triple height and double width & DS9X which is triple height and triple width. The downsampling modes are better at reducing flicker on shadows and stabilise moving images.
The in game MSAA based modes are as follows: MSAA which is 4xMSAA & High which is an SMS custom 4xMSAA mode (adds transparency anti-aliasing). The MSAA based modes are better at reducing flicker on vertical lines, like fence posts & lamp posts etc, but introduce white dots on edges (less so using High than MSAA).
There is one in game mixed mode which is DS2M. This combines 2xMSAA with DS2X. Sadly the although this does include downsampling, it doesn't reduce the flickering on shadows, in the same way that the pure DS?X modes do. It does introduce white dots on edges (as per the other MSAA modes), but to a lesser extend than MSAA (as it is only using 2xMSAA).