What you are talking about is Intel Smart Response SSD Caching. However, it wont solve your primary issue of not having enough disk space. The cache will only speed access to that disk space.
The technology did not catch on because it doesn't work really well. Performance gains were modest and did not justify the additional costs. It was designed when SSD drives were not big enough to hold the entire OS. That time has past and now you can get a SSD that will store your OS for around $100.
Their are two solutions to your problem.
First, and recommended, buy a new hard drive with sufficient space. Mirror your existing drive to the new drive and expand the partition using GParted or any other capable tool such as Partition Magic. You will now have sufficient space. If you want to get that SSD boost you were looking for buy a hybrid drive that will have some onboard SSD cache.
Second, buy the SSD, but rather than using it as you are thinking, just add it to your system. Once installed, you can move your Photoshop Scratch Disk to the new SSD. You may also consider moving your system PageFile to that disk as well.
One final note, and the reason First is First. Running a hard disk near capacity is a recipe for disaster. When a hard disk is near capacity it is forced to write large files into the nooks and crannies between other files already stored. This results in what is known as fragmentation. Fragmentation happens all the time but when a disk is near capacity it is worst case scenario. This fragmentation can become very severe and destroy performance. Imagine if you had a notebook of papers and you stored each sheet in a different room, shoved under a couch or bed. At the bottom of each sheet was written the location of the next sheet. This is fragmentation.
Making matters worse, with little reserve space on the drive it becomes impossible to defragment the drive. First things first, get some more space, then think about adding SSD.