Ah well. It was worth a shot (literally), but, like all Carmack's games before it, Quake 4 renders whatever hardware you may happen to own completely and utterly obsolete (even if the game is essentially Raven's, the rule is still valid).
On my iMac G5 1.8GHz (which is only 200MHz shy of the base requirements for PowerPC machines), it's somewhat playable at 800x600, but the frame rate drops tremendously during battle, so I completed the demo level running at 640x480 (which is, well, kind of blurry on a 20" LCD). Two and a half years have bumped the polygon (and, most importantly, shader) count tremendously, and even on this, last year's machine, the difference is very noticeable.
There's something else I hadn't counted on, though. When I last played Quake III Arena (soon after my little incident, just to see what it would feel like) I was calm, relaxed, calculating - the game slipped over me like a snug mitt, and it all boiled down to targeting and flying through levels by the seat of my pants (somewhat warm from all the rocketry). It has always been sort of a 3D ballet, and it actually relaxes me.
Quake 4 is full of strange noises and surprises (even if scripted ones), and fighting through the dark corridors aiming at the blurry pixels behind incoming Strogg fire left me feeling a bit too excited and somewhat uneasy - not Doom III uneasy (I thought that game was downright scary in broad daylight), but still something I could do without.
I still get little aches and pains in my chest sometimes (not today, though), so I'll probably take it easy and keep looking at RTS or turn-based strategy stuff - preferably for the PSP, if Sony ever feels like taking a stab at that segment.
But if I get a MacBook Pro, I know what I'll load on it first to get a feel for the machine...