I'm using a dell laptop - an inspiron 9400, not that powerfull (it slows down loads when the polygon count gets too high)
Really, you just need to go through a couple of tutorials (supplied with zbrush) to grasp the basics, it took me a few days before I could sculpt something - it's like learning how to use photoshop for the first time, and is no where near as complicated as 3dsmax.
The only thing I'd like to point out is that it's allot more organic than something like max, the process of creating something is completley different. For the first few sculpts you will end up with something which will not look properly finished, something akin to what a kid would sculpt out of clay - a little messy!
Download the trial version first to see if you get along with it
Sean