The performance increase is huge because the OS reads and writes from the drive for tons of little tasks you do when you use the computer. Everything is much snappier and more responsive in addition to the lower boot/resume/sleep/shutdown times.

If you're only putting your OS on the drive, it can be very low-capacity, but I'd strongly suggest installing your programs to it as well because that's one of the biggest performance increases you'll get with an SSD. Games can go on the drive if you want, but games are also huge and usually why I see people opting for more expensive high-capacity drives. 64 GB should be adequate for an SSD for your programs and OS if you don't have any huge productivity applications like the Adobe Suite.