I mentioned this the other day, but I started out with the goal of writing my thesis on PowerPC hardware.
As the size of the document and the number of citations grew, I found it increasingly more difficult to actually accomplish this. It was taking me 2-3 minutes per citation on my Quicksilver, and it's frankly time I don't have.
I think that at the root of the issue is that I'm using two crummy pieces of software-Office 2008(specifically Word) and Endnote X4. I need XML compatibility to too great of an extent to use anything earlier than Office 2008, and I have to much past work in Endnote to consider switching to another citation management program.
Even on my i5 MBP, it still takes Endnote about a minute to generate a citation, and I managed to crash Word(2011) twice within about two hours while working. As much as I'd love to keep using PPC for this, my time is too valuable right now to mess around with it.
In general, though, I find that I can comfortably browse the internet(I don't do Facebook, and use Mactubes for Youtube videos) on a G4 or newer. I use a G5 daily at work(2.0 dual core), and it handles everything I need to do on it just fine. For lighter word processing, I'm content using Office X and even Office 2001 or Word Perfect 3.5E under OS 9. Even a G3 or 604E isn't
that bad using OS 9 and Classilla.
I'm not a gamer, but still enjoy playing a lot of the games I grew up with. For the ones never written for Macintosh, I use DOSbox on my MBP. Otherwise, however, where a Macintosh version exists, I play it natively in OS 9 on various G3 and G4 hardware. Honestly, I've found that one of the best gaming computer I have is my Clamshell iBook. I tend to keep disk images on the hard drive and mount them using Toast, which makes the games a lot more responsive than using the optical drive and also avoids having to switch disks(many 90s games required both installation on the hard drive and for the media to be inserted, and some had multiple disks that had to be changed at points in the program). I've found a lot of the stuff that I play to be more responsive on that than on newer or older hardware(interestingly enough, it's faster for some programs than even my 500mhz Pismo with 1gb of RAM). The only game where this really doesn't work-at least not without a numeric keypad-is Civilation II, where the numeric keypad is a virtual necessity. I play that on my iMac G3