For CAD applications you do not want a "desktop/gaming" GPU. You want a "Workstation/CAD" card.
The Autodesk "Certified" Graphics Hardware List (for WindowsXP/Vista/Vista-64)
Autodesk - AutoCAD Services & Support - Graphics Hardware List
The Newegg link to PCI-E format Workstation Graphics video cards
Newegg.com - Computer Parts, PC Components, Laptop Computers, Digital Cameras and more!
Basically, don't expect a gaming GPU to perform CAD work well, or a CAD GPU to perform gaming tasks very well. The applications are quite different.