Its a multi-tasking computer, thats why she'd want a laptop.
but for CAD you really want a desktop. My E6400 with 2 gigs ram and 7900GTO runs any CAD program superbly - nothing lags or anything.
Dual Xeons or Dual opterons would be stupid to buy, 4gigs of ram is stupid and couldnt be utilized...how many cad programs are 64 bit? None, SolidWorks is going to release a Vista version soon.