Hello!
A SSD will help the load times of most applications, and as I also use ACAD, you well know how large of a program it is. Also, the newer versions (2014 especially) require a lot of total computer resources.
As for how SSDs work, you'll be instantly amazed at how fast they are. They make loading everything from the OS down to every program almost instantaneous. However, the reason people don't do them for major storage (like music or video playback) is that the increase of it is very minimal, and that information isn't always kept for long periods of time; for instance, I'm sure you'll keep your ACAD installation far beyond how long you'll keep a random video.
The best thing for you to do not only for performance but also for a normal cost, is to just keep the 750GB drive where it is, and add into the second bay a SSD.
As for which SSD to get -- as far as I've experienced, you won't be sorry going with any newer version SSD -- Many of them are quite comparable to each other speed wise. One of the front runners being the Samsung 840 Evo. The Samsung 840 Evo just took a huge price drop today also, with Newegg and Amazon having a price war to see who would have it the cheapest. Assuming you are in North America, it's the best time to get the "top rated" SSD out at the moment, at an incredible price.
Your ending storage capacity: 750gb 7200RPM for Movies/Music/Your backed-up ACAD/ACAD Drawings (things that are okay with a "slower" drive), and 120/250/500/750/1tb SSD for your Operating System and Programs (things that are large, and need to flush a large amount of data to the RAM at fast speeds).
Edit: Yes, you can use an SSD for storage -- It's just extremely overkill. Also, there is a good sticky thread for using "ramcache", which will also "speed up your system". My usage is SSD for OS / Programs, HDD for storage, and Ramcache as a "buffer" for stuff being written/recalled from the HDD. But, let the pros on the sticky explain that to you.
