Well, this is a tough one. it really depends. What most people do is, get the best CPU and GPU they can, then get the Mobo and ram to match.
If you are doing CAD work, your priority will probably be CPU. Your budget isn't very high, so look to spend around $200 on the CPU. For the GPU, around $225-250. Now, find a motherboard that has the features you want, and is compatible with your components. (around $100-150) Lastly, the ram. I would spend whatever you have left, in this case, between $50-100 dollars. For that, you should be able to get some nice ram, that will serve your needs.
Hope this info helped!