For some games, you can just run Steam in Wine, and the game in Wine, and everything runs out of the box. Check the Wine documentation for each game before buying to ensure it works.
Yup, Portal ran perfectly for me on Ubuntu ( 9.10 or 10.04 iirc) that way a few years ago. Was brilliant.
Even though I don't game anymore, this still makes me very happy. A big step for Linux on the way to becoming a first class citizen in gaming, one of the last big moats to its general acceptance.
