All SSDs are not equal — not by a long shot.
Some SSDs (but probably not any of the ones that are comparable in price to a 320GB hard drive) are substantially faster than any hard drive. Most are somewhat faster. All are more shock-resistant, but their longevity in real life is still unknown; they are too new. They are outrageously expensive on a per-gigabyte basis now, being new, and the really fast ones cost much more than you paid for your computer. Those SSDs are all Serial ATA models that your Powerbook can't use anyway. There are few PATA SSDs in production now. For user reviews of them, try the Drive Compatibility Database at
http://www.xlr8yourmac.com and the websites of such vendors as newegg.com, buy.com, zipzoomfly.com, macsales.com, etc.
Don't count on longer battery life with a SSD. Boot time will be shorter, but there's no reason to reboot a Powerbook except when a software update requires it, so that's really a non-factor. Application launches will be faster. By far the best reason to buy a SSD is for the extra shock resistance, if your computer will be transported a lot under rough conditions or operated while you're riding in a car, bus, train or plane. You'll have to decide how much that's worth to you — figuring in also the cost of a second external hard drive on which to back up your first external hard drive, and the inconvenience of sometimes having to carry an external drive with you just to have access to all your files.