GDDR5 has twice the bandwidth per mhz. So ATI's 256 bit memory bus is, on paper, the same as Nvidias 512 bit bus, but nvida's is much more expensive. Also nvidia and ATI dont make their own RAM, the graphics card makers buy the RAM, the GPU (from ATI or Nvidia), and the PCB and put it all together.
SLI\Crossfire is the ablity to use multiple graphics cards to produce one image (on one monitor). SLI is nvidias version, the latter is ATI's
More VRAM is not always better, it varies between game to game. There is just too many varibles to give a clear answer. If your buying a graphics card VRAM should not be the first thing you look at.