There is a better option. OCZ has come out with something they call the Synapse. It is an SSD that acts not as a boot drive, but as a humongous "cache" drive for the boot drive. Essentially, it uses software to borrow some power from the CPU to turn your entire spinning hard disk type storage drive into a gigantic SSD.
It makes perfect sense, too. While windows, all updates and your favorite programs could easily overwhelm even a 128GB SSD...
The files actually USED on a regular basis are probably closer to 10 or 20GB, at most. The software keeps track of what files are used and stores copies of those on the SSD cache drive.
Thus, even a measly 64GB SSD cache drive can give you the performance increase of (for example) a 512GB SSD drive.
I mention OCZ Synapse, as that's the one I use (windows and many programs boot fully in about 10 seconds!) But I believe Crucial makes their own version of a SSD cache drive, also.