Problem with upgrading is that the age of a PC has a significant effect on the price of components. Upgrading parts on a PC using current technology is usually fairly easy and cheep. But older machines almost always need parts that are no longer in mass production and as a result fitting new becomes ludicrously expensive and not worth the effort. The best option would be to check out e-bay or a computer fair.
As to the viability of upgrading your machines, post the specifications and weigh the different opinions you'll get.