Ok, time for a little summary of my "Steam for Linux"- / "SteamOS"-testing:
All of my current Linux-based Steam-games (50 out of 67) run flawlessly on both Linux Mint v13 (with backports) and Ubuntu Desktop v12.04 (vanilla).
- They do not require a top-of-the-line gaming graphics-card.
- They (also) do not require vast amounts of memory (8GB will do fine).
"Linux-gaming" Google+ photos:
https://plus.google.com/photos/104557379246577572135/albums/5977405744473413313
SteamOS is still in it's early testing-stage(s). You basically have to posess a fair amount of advanced Linux-tinkering skills to be able to utilize it properly, but things are progressing. There are some automated versions being made (custom ISO-images) to let anyone install, test and even debug it on hardware (without the earlier requirement of a EFI-capable motherboard, you can manage with a BIOS-version now).