I've got a 980 GTX running on Ubuntu 14.04.3 and it's pretty good overall. I can play most games at a decent framerate in very high quality @ 2560x1440. It can run Portal at 4K, no problem. The rig I have is pretty new and cost just over £2000 though. I've had less impressive results with lower spec'd hardware. As with all things gaming/hardware related, YMMV.
Yeah, Portal isn't too demanding. I've got similar hardware and even it strains with the more recent higher end titles, whereas Windows runs it beautifully.
Until the driver situation improves a bit, it's going to be hit-or-miss on a per-game basis.
