Steam is the best thing to happen to non-console, non-mobile gaming in a couple of decades. If the resolution of the host display is set in-game to the same as the client, that's that much less work the client has to do. It sounds like as much of the work, including graphical work, as possible should be done on the host. Remember, the client can be a low-performance device, so long as it can sufficiently play the stream, as it doesn't handle rendering the game's graphics. Steam, for example, shows a little Apple logo next to every game that. While it would be feasible to do this on a headless server with a beefy processor and minimal graphical power, you're almost certainly going to get better results if a decent enough GPU is used on the host. First, let's get the obvious out of the waythere are plenty of Mac-compatible games out there.
I could be wrong on that, but I'm pretty sure that's how it's always been. Ideally, the hosting machine would need to be powerful enough to play the game and to encode the stream simultaneously. It only comes with a few levels and to play the rest of the game you need to actually own Quake II on steam. Does the hosting machine need powerful graphics or would a headless Xeon PC server work well? I ask as I am looking at getting one anyway to host opensim and that only being a database doesn't require much in the way of a GPU the graphics being handled by the client viewer.
