Well, are you sure you even need a mini? They're great machines once you add some RAM to them (I just got one two days ago and between the RAM-starved stuttering it's pretty fast), but you said you have an iBook and you just need to run your 19" monitor at a higher resolution. Well, there's an
easy hack out there that lets you run external monitors at higher resolutions on the iBook.
If all you need is more screen space, the spanning hack should do the trick. But if you need an actual second computer, then the mini sounds like a good choice. I'm not sure if you'd really need the 1.42 GHz, seeing as how it only has a 14% clock speed increase over the cheaper model. But get whichever you want. Also consider installing the RAM yourself - a 512 MB RAM module is around $50 now, and a 1 GB is about $130. The savings is small if you go with 512 MB, but it's huge if you decide to jump to 1 GB.