I've got CS, CS2, CS3 (all upgrade versions) and other bits and pieces lying around. Can I do something useful with them, or do I just dump them?
You cannot do anything useful with them. Still you mustn't dump them all; keep at least one, or you'll void your CS4 license.
By the way, you also need to own at least one full version of an older Photoshop version (as opposed to an upgrade version), or your current license is void. An upgrade license is valid only when you have something to upgrade from. After having upgraded several times upon the original full version, you still own exactly one
license, not several, and you---formally---will always need your old original version along with the upgrades in order to be able to prove legitimate ownership. The absolute minimum you should do is to write down all the serial numbers of your original full version and all the upgrades and keep them in a safe place.