Here's a quick run-down, as a Linux gamer:
* Steam for Linux is pretty rock-solid, at least on the common desktop environments. Some bugginess once you get into bare window manager territory (particularly WindowMaker or anything that's designed specifically for tiling), but otherwise works without issue.
* A very large number of indie games are targeting Linux either alongside or soon after Windows. Kerbal Space Program is the prodigial example here, and seems to be leading the charge for Unity3D.
* Pretty much everything made by Valve now runs on Linux natively, for obvious reasons. A part of me hopes that - when GabeN gets over his fear of the number "three" - Half Life 3 will launch on Linux/SteamOS first.
* Some non-Valve big-budget games and publishers are gradually making their way to Linux - both new and old. Borderlands 2/PS, much of the Tropico series, Cities: Skylines, Serious Sam 3, The Talos Principle, KoToR II (no KoToR I yet, alas, but since it's the same engine, there's probably a good chance of that changing soon), Civilization V/BE, various others.
* SteamOS is, in my experience as of late, excruciatingly difficult to install; Valve, in their wisdom, ships it as some zip archive instead of a more typical DVD/USB image. Perhaps the SteamOS installer is meant to be written to a USB stick with Windows rather than with Linux? Whatever the case, I've yet to get SteamOS to actually install, though I'm planning on revisiting it soon.
* Single-monitor setups are generally fine. Multi-monitor setups become problematic, since the games tend to do wacky things with their resolution and fullscreen-window sizes; I generally work around them by running games in windowed mode.
* Which graphics card vendor to go with depends on whether or not you want to stay strictly on the free software path. AMD's cards are currently vastly superior to Nvidia's when running on FOSS drivers, in no small part due to AMD actually participating in said driver's development. Nvidia's cards are currently superior to AMD's when running on proprietary drivers, though this might change once AMD migrates from fglrx (the current non-free driver) to something based on the FOSS "radeon" driver.
* Wine is quite fantastic nowadays, though game compatibility is still hit-and-miss. I regularly play Fallout: NV on Linux+Wine with very few issues (namely, the occasional crash after a few hours of gameplay); Fallout 3 is playable, but you can't use the Pip-Boy radio due to a bug with Wine and DirectSound (?) (sound will be garbled, and the game will crash with a deadlock after a half-minute or so). Valve's games are naturally very Wine-friendly (though this is no longer necessary, of course). Steam works reasonably well, but requires installing some font packages in order to get text to show up. Older games tend to work better than newer games.