If it's strictly for rendering, you don't need a graphics card at all, as you will just be rendering stuff.
If your rendering SW supports CUDA or OpenCL then a graphics card will provide a good amount of extra juice.
However if you are refering to 3D design using real time 3D rendering then, AMD's FirePro series and NVidia Quadro series is the way to go.
SLI or Crossfire is generally not useful for it, you just want a card with plenty of video ram and good support for Open GL and that's what the above provide.
Their prices also go up to about $4000. ( like this:
Nvidia Quadro 6000 )
or $3000 on the AMD front. ( like this:
ATI FirePro 9800 )
If a consumer model will do, then the GTX570 for example is a decent card at a much more reasonable price in comparison.