WARNING: These are my personal opinions as a former system builder/gamer/drafter.
You should consider the following carefully when looking at upgrade options.
1. If your system still has room for more RAM, you should consider maxing it out if that is in your budget. Try to match the RAM (so all the sticks are the same), as that helps with system stability.
If you are running VMs and Autocad, every little bit will help.
2. When you upgrade your hard drive, if you can use a Solid State Drive (SSD) you should consider doing so, they are a lot faster and, of course, more expensive, as they are essentially just a bunch of RAM.
Many users have a SSD as their boot drive to improve performance, and a standard large Hard Disk Drive (HDD) to hold a bunch of data, but again, that is a more expensive option.
If you cannot use (or afford) a SSD, look at the largest (affordable) drive that is compatible with your system, and choose the highest spindle speed/largest cache as those will help improve performance.
3. If you are buying online, make sure you choose a reputable vendor like Newegg that has a great reputation and a good return policy (just in case).