The amount of video and system RAM is gonna be the key there.
For CAD, I'd say your minimum that's "do-able" would be 256-512 megs of RAM, and a high end gaming card with 64 megs of RAM, Geforce2 or Voodoo5. If you can wait and money isn't an issue for this, the next Voodoo with 128 megs of RAM (and external power supply ::drool:

should probably do the job.
Xayd