If you're going to game under X, it's crucial that you know a bit about X. The "X Window User HOWTO", and especially "man XF86Config" are required reading. Don't short change yourself; read them. They have an extremely high "information to space" ratio. Many problems can be fixed easily if you know your way around XF86Config (or XF86Config-4).
Whether you're trying to diagnose an X problem or requesting help from a mailing list or Usenet newsgroup, you'll want to have as much information available as possible. These are a set of tools you can use to obtain that information.
One of the best diagnostic tools and sources of information about your X system is probeonly output. To use it, kill X if it's already running and from a console, type:
X -probeonly 2> X.out
Yes, that's a single dash; so much for standards. The output of X goes to stderr, so we have to redirect stderr with "2>" to a file named X.out. This file will have almost everything there is to know about your X system. It's crucial that you know the difference between the various markers you'll see in probeonly output:
(--) probed (**) from config file (==) default setting (++) from command line (!!) notice (II) informational (WW) warning (EE) error (??) unknown.
Here's an example of some information I gleaned from my output:
I'm running at 16 bpp color:
(**) TDFX(0): Depth 16, (--) framebuffer bpp 16
X has detected what my videocard chipset and videoram are:
(--) Chipset 3dfx Voodoo5 found (--) TDFX(0): VideoRAM: 32768 kByte Mapping 65536 kByte
xvidtune is your friend when your X screen is shifted a little bit too far to the right, or if the vertical length is too small to fit on your monitor. However, it's a great diagnostic tool also. It'll give you:
the hsync/vsync range specified in your XF86Config file
the 4 horizontal and 4 vertical numbers which defines your videomode (the 1st horizontal/vertical numbers gives the screen resolution). These 8 numbers will tell you which modeline your X uses. See the XFree86 Video Modetiming Howto for more information.
the "dot clock" your videocard is running at.
xwininfo tells you all sorts of information about X windows. And actually, your "background" or "root" window is considered a window too. So when xwininfo asks you to click on the window you want the information on, click on your background. It'll tell you things like screen and window resolution, color depth, window gravity state (which gives a hint to the window manager about where to place new windows), backing store usage and more.
xdpyinfo gives cool stuff, like X version and loaded extensions (invaluable when trying to see what's missing, like GLX, DRI, XFree86-VidMode, etc.).