T42's don't have a sata interface so an SSD upgrade would only be possible with a narrow bunch of fairly slow and expensive models that don't stack up compared to their sata counterparts.
The performance increase I can not say, depends on what you use the laptop for.
An upgrade to a standard larger capacity western digital drive would be possible and more then likely would still make a nice performance difference compared to the drive that came with this model laptop.
There is a bunch of things to look out for, depending on your operating system, BIOS, etc........
For the price of an decent size SSD that would work for your system you could buy a T61 with windows 7 & fingerprint reader etc for $100 more on ebay. or a good netbook with slightly smaller screen & SSD w/ warranty all already there might be fine too depending on your needs.
Just what i might do, even thou not part of your question.