<a href=”http://www.linux.org/dist/”>Linux Online</a> is a good source of information for obtaining linux. You can probably find some help on choosing the right version there as well.
If you want to beef up storage space or space for the OS, you can always buy an external harddrive and boot from it.
Another great source to find linux distros is <a href=”http://distrowatch.com”>Distrowatch</a>
You MAY be able to purchase an external hard drive. If your laptop has USB ports, which is pretty unlikely. Even then, you would most likely need an external hard drive that supports USB1.1, even on an old, slow system that will CRAWL.
With regards to distributions, I would go with Xubuntu or Puppy Linux. Those are the only two lightweight distros I have experience with, and they were both noteworthy.