Do different desktop environments affect game performance?
Solution 1:
A few months ago, Phoronix posted some benchmark results of 3D performance with different desktop environments. You can find the article here: http://www.phoronix.com/scan.php?page=article&item=ubuntu_1204_desktops
Do note that these were results from Ubuntu 12.04 so results may be a bit different in 12.10 or later. As you can see from the results, Unity tends to eat away a little bit more then other DE's. So, if you are going to make a system purely for gaming, it looks like using the Xfce (Xubuntu) environment will provide the best performance.
Solution 2:
In general the problem with gaming is the graphic card (it's a simplification, I know). Usually processor and memory aren't the bottlenecks. So the usual way of checking consumption is not really appropriate for this problem.
The real impacts comes usually because modern environments use graphic processing for handling the windows. This allows all this transparencies, previews, etc. But for games this is a major problem as it adds a delay in the processing.
All the environments are working to try to detect games and deactivate their "3D processing" on full-screen games.
It is a work in progress and each one is improving.
So in general the bigger consumption of a "bigger" desktop environment shouldn't be a problem but currently it might be.
For 12.10 this benchmark could give you an idea of how are the things. But this things are changing really fast. Take a look on this review:
http://www.phoronix.com/scan.php?page=article&item=ubuntu_1210beta_desktops&num=2
They are checking different DE and including a bleeding edge improvement on KDE.
You should think if the differences are important for you (they aren't that big in most of the tests). And also you can install ubuntu for standard day work and install Lubuntu for gaming with:
sudo apt-get install lubuntu-desktop
But keep tuned. This is changing fast!
Solution 3:
Yes. You need to consume less resources if you have less resources. That goes without saying. This is what Lubuntu was designed for.
At the same time, if you have a more powerful system, you might have other motives for being frugal and might simply wish to free up your system resources from running a powerful desktop environment.
You can run all the same software on Lubuntu as Ubuntu and you should notice a marked performance increase etc. The only downside is a more primitive desktop environment, which not everyone appreciates.
In my own experience, there's nothing better than a light-weight OS such as Lubuntu or CrunchBang on a powerful system for performance. Of course, the spartan feel of Lubuntu is not to everyone's tastes. But you'd be suprised at how pretty people can make it.