It may hold your OS at first. But it might get a bit laggy and unresponsive down the road because your system will eventually eat up a lot of space in the disk.
Just to provide another perspective, I've been running a 40GB SSD as my boot drive since mid may. My usage looks like this:
9.4GB Applications
5.2GB Users
2.5GB Library
2.1GB System
2.9G other used space
Of course, my 22GB used excludes the hibernation file which is the same size as installed ram (i turned it off) and movies and music which are on my mechanical hard drive.