SSD's of 60Gb and below have only one real advantage, that being used in Intel Smart tech as a cache drive. Basically it's a giant Windows temp drive that runs everything through it at SSD speeds, then writes the saved data back to the Hdd when it gets a chance. Seagate has already followed this direction with the advent of the Hybrid drive, a HDD with a built-in SSD. It's a good compromise, but still middle-of-the-road, faster than a standard HDD, but overall slower than a SSD. For a basic pc with only one or two large, often used programs, a 120/128Gb SSD is fine. If you are planning on much more than that then a 250/256 Gb SSD would suit better. Whatever size is chosen, never load them anywhere close to full, the remaining space is used by Windows etc for its temp working space, and without that space you'll find the pc gets very slow, very quickly.
Setting up a 60Gb as a cache drive will require doing from the start, its not an additional drive as such, but an add-on to an existing drive. If that's a possible direction, even a 32Gb maybe a cheaper option, yet retain the advantage of the SSD. Otherwise, a 60Gb would pretty much be a waste of money as a Windows only C: drive.