I put the hybrid disk mentioned by C3Sound in my 2007 MBP. It's a Seagate Momentus XT 500 GB with 4 GB flash. It runs all right and is quite a bit faster than the stock drive, but it's not the holy grail that the manufacturer wants you to believe it is:
- You have no control over what goes on the solid state part, that's automatic (the files that are used the most go on the 4 GB part).
- There are some problems with the drive spinning down at the wrong time, and then immediately spinning back up. This causes the interface of my DAW to freeze for a few seconds, though audio continues without interruptions. This is supposed to be fixed with a firmware update, but I haven't tested that yet because the update tool is windows only.
- It's not nearly as fast as a proper SSD, say a Crucial or Corsair Vertex or Intel drive.
- It actually uses more power, generates more heat and makes more noise compared to the stock drive, because the stock one runs at 5400 rpm and this one at 7200.
So it's a compromise between storage space and speed for a low price. If you want speed and battery efficiency, I'd suggest a proper ssd, with an external 2.5" 5400 rpm hard drive for storage.