I would definitely suggest an SSD because of its insane performance. Stock drives are considerable slower and Velociraptors use much more power and have a much higher failure rate. Also, with SSD speeds, your system will boot much quicker and you applications will load much quicker as well. I don't really know much about the write speeds of velociraptors versus SSDs, but I do know the OCZ Vertex 2 has sustained write of up to 250 MB/s, which is much better than Intels x25-m of 70 MB/s. Also, the read speeds of the OCZ SSD is 285 MB/s and Intels is 250 MB/s.
I am considering an SSD as a boot drive and will be moving my home directory to a separate drive and only have OS and apps on the SSD for quick boot times and app load times.
OS X has 4 main folders: Applications, Library, System, and Users. When you move your home directory, you are moving the main part of the Users folder, which is where a lot of the storage is. The little icon with the house in finder is your home folder.
When I get my SSD for my boot drive, I will move the home directory and keep the Applications, Library, and System files on the SSD. A few days back, I went to see how big this would be, and mine is only 43 GB and I have quite a bit of applications, including the CS5 Master Collection, Aperture 3, Maya 2011, ZBrush, along with many many others. So really, if I were to get a 256GB SSD, that would be way overkill.
For you, I would see how much space those three folders are and that might help with your decision.
This website should help explain how and why you would want to move your home folder.
As for physically putting it in your system, many people like to use something called an
ICY dock. However, some SSDs come with a 3.5" adapter or something like that. Also, you could put this in an empty optical drive in your MP and save the other 4 bays for mechanical drives, something I am considering myself.
Hope this helps in your decision making and good luck!