Depending on how large your CAD files are, having your ACAD and the files on the SSD will dramatically help you speed up your use.
When dealing with large CAD files that have 100's or 1000's of XREF's, MODELS, etc... they can take minutes to load. Not to mention minutes to save (which I'm sure you do as often as I do). Having all of the files on the SSD will cut down on this a lot. If you are running into issues where you are running out of space, you can always move a project over to a storage drive once it has been completed.
I purchased a 60GB SSD awhile back and noticed a huge difference in performance when I was using all of my CAD files off of it. I just wish I would have purchased a large one at the time.