The proposed spec looks fine. As for the graphics card, unless your sister plans on doing a lot of 3D work, you can pick up a good graphics card (minimum 128MB vidRAM) for $250. There's no need to spend more on the video card than you would on the rest of the parts. Any extra $$ can be put to better use by investing in another 1GB of RAM.
Recommendations for video cards can be found online at CADalyst Magazine. You might also consider buying rather than building a machine. Try Xi Computers or IBuyPower. The first is a dedicated CAD computer manufacturer while the second is geared more towards the gamer.
Gamers and CAD users tend to have very similar needs and generally push their machines harder.