Hardware :: In What Ways Does Size Matter With Ghz, GB Ram And GB Hard Disc?

I'm afraid this reveals really basic ignorance on my part but can someone please tell me what aspect of a computer's performance is affected by having more or less Ghz, more or less GB of Ram or more or less GB of hard disc memory? For instance, if a choice must be made is it better to have more Ghz or more memory (and if the latter, more Ram or more on the hard disc?) As a concrete example, I have recently acquired an iBook with 1.2 GHz, 1.25 GB RAM and a 30GB hard disc. I intend to use it mainly for web browsing, email and word processing. I undersatnd that the Ghz and RAM cannot be increased but I could have the hard disc upgraded to say 160GB. But what would be the effect on performance, and would it be worthwhile?