That's a key factor that most people, even a lot of people here on this forum, don't have. iBooks aren't maintenance-free now that the vast majority of them ever built are eight or more years old -- and truth be told, most of them were built poorly anyway, suffering from a variety of board issues in various generations. iBook maintenance skills are something I don't presume everybody who looks at this forum has, which is why I tend to recommend slightly newer wintel laptops that are designed to be hauled back and forth across the globe by salespeople. They do very well in the hands of all people, because they're built to withstand almost anything.
Also, for somebody who already has an iBook and it works, sure, keep it and keep using it -- but for somebody who doesn't already have an iBook and doesn't have iBook repair experience? Stay away, because the iBook was meant to be a short-lived consumer computer.
That's just a cute fluke, and a 2008 Mac Mini can still be upgraded to hold 3 gigs of ram. (Which I'm surprised you haven't done, but that's just me and I tend to want as much memory as I can get in every computer I have.) I once had a Quadra 840av with 48 megabytes of ram, and a 233MHz iMac with 32MB of ram. The iMac was still faster even though the Quadra had more ram. Likewise, I bet a whole slew of factors lead up to your mini being faster (at everything) even though it has less ram.
That's unfortunate. Everything I've heard about the D-series Latitudes is that this is a fluke, but I'll admit to having used relatively few 600s and 610s over the years. The 620s and the 630s that have integrated graphics are completely rock solid. (The 630s with discrete graphics have the nVidia graphics with the bad soldering -- not bad machines, but they'll potentially need to be repaired in their lifetimes.)
The E-series is also solid, from everything I've heard.
This all presumes the barest modicum of respect for the machine, of course. Any laptop is going to have its power connector fall off the board if you exert enough pressure on the power connector, and any laptop is going to develop issues if you put it on a carpet and never clean dust and fibers out of its ventilation ducting, and any laptop is going to develop a bad disk if you drop it on the floor enough times.
Without considering software at all, my recommendation is almost always a Latitude or a ThinkPad -- they're more flexible systems in terms of expansion and options, they're often available for less than a Mac notebook, especially if you're looking at used/refurbished/off-lease machines such as from geeks.com or dfsdirectsales, and there's often a way to get a better LCD panel, not to mention nice little things like the epic ThinkPad keyboards or the ThinkLight, which I still prefer over a backlit keyboard any day of the week.
But if Mac OS X is a top priority for you, then buy a MacBook, but be aware that you're comparing a budget-conscious consumer laptop (with relatively well-documented issues with its casing) to the top tier of corporate fleet laptops, often with no known defects or weaknesses, just depending on the model and vendor you go with.
Another option if you're just looking for something to run modern apps is to get a desktop -- if you're not taking a bunch of in-person classes or roaming around the world with your computer, a desktop is almost always going to be cheaper, faster, more durable, longer-lasting, and easier to repair if something does go wrong. Even I'm guilty of not giving the desktop quite enough credit, in this modern world of going to a cafe and using somebody else's horrible Internet connection, or taking your laptop to the sofa.