I would update in this order:
1. Video card
2. CPU
3. RAM
4. HD
A good video card with strong CAD drivers has been (in my opinion) the best investment at about any level. It takes load off the CPU, can regen faster, and some cards have nice AutoCAD zooming features. We have been using the Nvidia XGL line and have been real happy with it. I know Nvidia has a FX line, but you would have to weigh out the benefits for the added cost.