I think this is what you are looking for
OCZ Synapse Cache SATA III 2.5" SSD - OCZ
I have been looking at getting one of these for my laptop rather than having an SSD as a primary drive. and having limited space.
These seem to be very good at their job and are getting very good reviews.
You add it as a second drive and it caches all of your regular applications and files for quicker access
It is like a better version of the seagate momentus hybrid hard drive
As you get better speeds from the SSD and a lot more cache.
If you install this you wouldn't need another SSD as your primary drive and dont even need to do a reimstall of the OS, I think you only need to install the OCZ software to allow it to function.