I like seeing how low-end of hardware I can get to play the games I like. Unreal Tournament (from 1999) and Tremulous are the 2 games I test the most often...
Well, right now the lowest spec'd system I've gotten to play Tremulous is a 700mhz AMD Duron, 384MB PC100 SDRAM, with a 64MB ATI Radeon 9200 SE OC'd from the stock core clock of 200mhz to 250mhz, and stock memory clock of 165mhz to 250mhz. Has a 80mm fan strapped to it with twisty ties, too. I tried other cards, this is the only one that worked/worked right.
I tried Ubuntu 8.10, and Windows XP. Ubuntu 8.10 made it unplayable. Might have been OK if I knew how to OC the GPU under ubuntu.
On windows, I didn't try stock clocks, I immediately OC'd it. Under default settings for that system(on Tremulous, a FPS based on Quake 3's engine), I got at lowest 20FPS, and at max 90FPS. the map I normally play on didn't drop below 30FPS.
Ubuntu, with stock clocks, got 10-15 FPS. Simply unplayable.