I strongly believe that DirectX would perform excellent with Autocad 2008 based on some research on Autocad's requirements and the 8800's specs which also would include DirectX's excellent design. With OS Vista, there won't be any problems! Just take note that Autocad 2008 requires a lot of memory usage and you should take steps in ensuring that you have at least 2GB on your mainboard.
Quadro, on the other hand, is exclusively made for developers. So selecting this model would be a great choice too.
Hope this was helpful!