the advantage of online storage is that you can access it from any pc
the disadvantage is, its not as secure as a back up CD or external source, also it can take a while to upload (downloading however should’nt be a problem)
as for sites, the best ones are not free, you pay for the services, if you’re looking for a really good free one, then you won’t find one if you wanting to store lots of data
however for your needs, i would have said your best option would be just to partition your hard drive… I myself keep my documents, things i need in one partition and the operating system along with application data on another, so when it comes to a format, my work is easily saved without the worry of having to transfer it from place to place