I discovered* a very useful set of easy to understand references to tweak Windows XP, Firefox, Nvidia Forceware graphics cards, various games, and many others at TweakGuides.com.
The two that I have spent the most time with so far are:
Firefox Tweak Guide containing almost everything you wanted to know about what, why and how to use and customize Firefox and then some. I already see a few changes I am going to test...
Windows XP Tweaking Companion (the downloadable free version). I just started reading this one, but if it is anything like the Firefox tweak guide, I expect I will learn a whole lot of new things.
* Thanks for the find go to adot's notblog