Don't take my writing too serious but what hardware is Steam4Linux supposed to run on?
My P3-Dualcore@1,33Ghz doesn't offer PAE so Ubuntu doesn't run.
My P4@3,2Ghz offers PAE but its Geforce 6800 - although technically within specs - fails starting TF2 because of some GL-extension missing. As the 6800 is the best native AGP solution available this is a dead end. At least it runs Penumbra although every level change will reduce FPS by 90%.
My Core-Q9550@3,4Ghz with its Geforce 260 is technically speaking just fine but officially I may only use Ubuntu 32bit and waste half of my memory (yeah, easy to work around) but still I need the uttmost updated bleeding edge drivers just to move the steam window around. Ayeah, 3D-unity and Steam hate each other. And every 3D game hates 3D-unity and Steam at once. So better disable 3D unity and close steam before launching the game or you will have 5fps.
My i7-3770K and also its Geforce 670 are too new for Unity. Couldn't get both running useful.
The only system running out of the box (mostly - WLAN runs better with hand compiled driver) is my netbook EEE 901 from 2008. Oh but I might mention that Steam needs between 10 and 25% of CPU even with all windows closed and doing nothing at all. So better learn to use cpulimit or your battery will be empty in no time.
Still its an interesting experience.