hey, Tangle, be of good cheer 
I also had that ‘brain strain’ feeling when I first got ZBrush :o – but don’t feel that you have to master every nook and cranny on the first pass – it can be used different ways by different people and you can have fun at all levels of mastery
also keep in mind that there are some technical functions included that are needed by people who make models for games or movies which might not be needed by hobbyist artists like us – you can skip those the first time around 
aside from going through the manuals and playing around (and the ctrl-click popup help), have a look through the FAQ on this site, esp. “why can’t I edit my model anymore?”
a Google search for “glossary 3d terms” might turn up some useful results – sorry that I can’t think of any sites, but I was in the same position regarding some of the terminology – much of that is explained in the 1.55b manual (e.g. “specularity”)
it’s important to know that “texture” in this context does not mean bumpiness or how something would feel to the touch – it would mean the image or pattern that is wrapped around or applied to a 3d object
“displacement map” – this is a grayscale image that is applied to a 3d object and which causes bumps or other things to seem to stick out of the object upon rendering, with white values sticking out the most, black not at all, and gray values proportionately … this is all usually scaled by some other “maximum displacement” setting
e.g., if you had a white ping-pong ball object and a displacement map applied to it that was black with white letters that said “blerg” and some little gray stars sprinkled around, when the scene was rendered, the word “blerg” would protrude from the surface, and so would some little stars, except not quite so much, and the ball, stars and “blerg” would all still be white
hardware: I think the demo would give you a good idea of how well ZBrush would work with your current system – also, I think the amount of RAM might affect performance more than the video card (?)
another tip: every tool (brush or object) remembers some other settings (e.g., stroke, texture and alpha), so if you know you’re going to change tools, it’s pointless to change those others beforehand, i.e., change the tool first and the others after
Welcome to ZBC, and have fun!
