Well it all depends on what type of CAD apps you're working with.
For anything like Autocad or light 3D, any ATI or Nvidia standard cards from the mid range will do. If you're in the more heavy stuff, high end 350-450$
high end cards will be just fine, but won't be "optimized" as much as dedicated workstation cards like the Firegl's or Quadro FX line of cards, but will give you decent performance none the less. I personally work in Structural steel detailing/modeling, and I build all the systems we use, and we recently updated our workstations and i ended up using 7600GT's 256MB i the Pci-e flavor, and notice that going from ATI to Nvidia, the "shadings" where less accurate, but that might also be because of the drivers, not the hardware itself...Apart from that, we work with models that contain as much as 7 to 8000 individual parts, not counting individual treatments of these parts without any lag or slowdowns when viewing the models in 3D (using an OpenGL engine).
Also as far as i know, one important thing to keep in mind is the customer service that comes with the "pro" lines of GFX cards. If you get a hardware failure of some kind , they will move a lot faster to exchange or repair your parts compared to standard gaming GFX cards, also drivers are heavily optimized for specific 2D/3D apps, so right off the bat you get better results, hence the price premium over "similar" cards from the gaming range.