The OS is not as relevant as you think here.

The primary arbiter of graphics performance (which I'm assuming is your concern because you posted in the OpenGL forum) is not the OS, but rather the graphics hardware and it's driver.

...Does this sound like a good move?

I'd consider it a toss-up.

Offloading the desktop environment to the GPU makes a lot of sense, because otherwise it's going to tax the CPU. For performance of the OS itself you obviously want to keep CPU headroom for number-crunching and other such work, so having the GPU handle display tasks can actually make your OS faster.

Raymond Chen has a good writeup of this for Windows here: http://blogs.msdn.com/b/oldnewthing/archive/2013/03/27/10405554.aspx and it's worth quoting as it will apply to other compositing desktop environments too:

Starting in Windows Vista, a lot of visual effects were offloaded to the graphics card. Consequently, the impact on system performance for those visual effects is negligible, and sometimes turning off the effect actually makes your system run slower because you disabled hardware acceleration, forcing operations to be performed in software.

For gaming, any desktop worth using is going to switch itself off when you go fullscreen - worst case is that it's going to keep a few MB of buffers hanging around, but it's not going to be compositing in the background behind a fullscreen game.

So rather than making a judgement based on what seems the "lightest" desktop (which may well give you a slower OS as you may end up losing hardware acceleration) you should make a judgement based on whether or not it has these performance characteristics I mention. Does it satisfactorily offload to the GPU in normal usage? And does it switch itself off when you go fullscreen in a game?

I might be incorrect, but I always assumed that non-3D emulators were mostly software-rendered anyhow. Most of the old arcade boards and such had some fairly oddball (moreso, by today's standards) graphics hardware and I'm not sure how much of it translates well to being GPU-accelerated.

Well, those hardware specs are better than mine, and I develop the Unity DE, so I think you'll be fine.

Keep in mind that when you're running in fullscreen mode with Unity you're going directly to the hardware: compositing is turned off. Use a lighter DE if you wish: Lubuntu or Xubuntu do not use a compositor, but you won't see much difference in fullscreen mode.

Don't bother looking for the proprietary Intel drivers: they're a part of Mesa. Intel open-sources their drivers and contributed them to and maintain them as a part of Mesa. Mesa is not just software drivers.

Xfce = nice. LXDE = not nice, and no longer developed (and LXQt is on development).

My two cents about lightweight desktop environments.