For a strickly gaming build I would go with the E8400 over the Q9300. For your uses, I would think a quad core will be more benefical for the CAD usage. Your overall best bet is to purchase a Q9550 over the your two CPU's. It provides the best of both worlds compared to the E8400 and Q9300... IMO
Just remember to review your motherboard and BIOS to make sure they are at the current revision to support your new CPU.