All right! Got the CPU (Core2Duo E6400 :-) at last and assembled the system.
It runs very smooth, even with transparency effects the CPU isn't showing any
extra load while moving windows around. That's something I never got with
opensource radeon drivers, even while using the binary blobs.
The only games I tested by now are Frets on fire (basically Guitar Hero for
GNU/Linux with a HUGE database of songs for download) and Openarena. Frets
ran better than on the old PC, I'd say like 60fps or so. Openarena wasn't
that good, in 1280x1024 it ran a little jerky, 30fps and below I guess. But
still very playable and maybe there is room for improvement there through RAM
upgrade.
For the sake of gamers using free operating systems, I suggest starting a
list of games that run with Intels cards (since nothing else appears to be
truly compatible with free software atm). Could look somewhat like this:
different tables for different types of videocards, on the left you have the
title of the game, on the top you have the CPU+RAM. This way almost every
possible configuration can be rated properly. 10/10 could mean 60 fps with
fullscreen high-res high-details. Everything below 5/10 would then mean
running under 30fps in small windows and/or some bugs.