(First, remember. It wasn't that long ago.)
Being fundamentally fed up with my RAM swindle and finding myself unable to get Fink to compile even the smallest package without a "bus error" or "segmentation dump" message, I emptied the loose change and ordered a 1GB DIMM from Crucial, just like Nuno. Shipping took 48h, since I don't keep regular hours and wasn't at home when the courier called.
First try at compiling OCaml (a unison dependency) went swimmingly (and this is not a trivial thing to compile - OCaml eats up a lot of memory), so it looks like RAM was indeed why my iMac was locking up randomly.
I will nevertheless stress test it a bit during the weekend, provided I can stay awake for long enough to use a computer.
The worse thing is that there are still casualties lying around: A couple of work documents I had edited before I figured out I had RAM issues are corrupted, one of them beyond repair (and the other only seems to have a couple of pages before ending mid-sentence with a Thai character). At least one JPEG image I had transferred from a test phone is corrupted too (apparently a borderline case).
I wonder what I will come across next...
Update: 6 hours, several e-mails, lots of SSH sessions, some SubEthaEditing with Davi and a revised Alcatel SpeedTouch HOWTO later, I have had zero lock-ups and a single application crash (VideoLAN barfed on a malformed video file, which I consider OK behavior for it). I also have the longest application dock ever, with dozens of running apps. Let's see if it stays this stable.