OK. After setting up my new iMac, I can say I'm thoroughly pleased with it. Everything feels snappy, but the 256MB RAM is definetly something I'll be upgrading, since any heavy usage (compiling things, several apps open at once, etc.) causes the usual BSD-like smooth deceleration due to swapping. Virtual PC is fast, but suffers noticeably when used alongside, say, iPhoto due to lack of RAM.
Exposé is a dream on this display, with windows remaining legible even when shrunk and silk-smooth animation.
On the more leisurely side of things, I've been down memory lane figuring out Quake III Arena console commands and cvars, and the settings I tried on my new iMac were as follows (all with 32-bit texturing, full detail, etc. - so-called "pro" quakers prefer to play in "watercolor mode", which is a combination washed-out mipmapping, vertex lighting and other tweaks):
com_maxfps 125 r_customwidth 1680 r_customheight 1050 r_mode -1 vid_restart
This yielded an average 80 fps (75 in heavy textured maps like q3dm6). Bumping down the resolution a bit:
r_customwidth 1344 r_customheight 840
...yielded 125 fps in space maps, less on heavy textured maps. the standard 1024x768 resolution froze the frame counter at 125fps solid (mostly, since my 256MB RAM resulted in occasional swapping).
I think I'll be going back to Urban Terror any day now... :)
It does seem a bit noisier than my 15", but then the CPU is an entirely different revision, and the GPU must be in there as well...