The depth and resolution are configured in the Section "Screen".

Depth stands for color depth or bit depth, is a computer graphics term describing the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. Higher color depth gives a broader range of distinct colors, but requires for larger frame buffer.

DefaultDepth integer
The legal value could be 8, 16, 24, 32.

When you specify your depth, X selects the bits per pixel (bpp) size in which to store the data. Below is a table of what bpp is used for each possible depth:

Depth	bpp
8	8
16	16
24	32
32	32

Resolution refers to pixel dimensions. Higher resolution gives higher display quality, but requires for larger frame buffer. The supported resolution and refresh rate for CRT are listed as following, modes up to 1920x1200 are supported for DVI as well.

Resolution	Refresh Rate
"640x480"	60,75,85,100
"720x480"	60
"720x576"	60
"800x480"	60
"800x600"	60,75,85,100
"848x480"	60
"856x480"	60
"960x600"	60
"1000x600"	60
"1088x612"	60
"1024x512"	60
"1024x600"	60
"1024x576"	60
"1024x768"	60,75,85
"1152x720"	60
"1152x864"	60,70,75
"1200x720"	60
"1280x600"	60
"1280x720"	50,60
"1280x768"	50,60
"1280x960"	60
"1280x800"	60
"1280x854"	60
"1280x1024"	60,75,85
"1366x768"	50,60
"1368x768"	60
"1360x768"	60
"1400x1050"	60,75
"1440x900"	60,75,85
"1440x1050"	60
"1600x900"	60
"1600x1024"	60
"1600x1200"	60,75,85
"1680x1050"	60
"1792x1344"	60
"1856x1392"	60
"1920x1080"	60
"1920x1200"	60
"1920x1440"	60,75,85
"2048x1536"	60,75

The resolution is set in SubSection "Display". For example,

Section "Screen"
       Monitor  "Monitor"
       SubSection "Display"
              Modes  "1024x768" 
              Depth  24
       EndSubSection
       DefaultDepth 24
       Device       "Device[0]"
       Identifier   "Screen[0]"
EndSection

Regarding to the modes out of this list, you can use the modeline to configure. Get more information from Part III. Chapter 6. Modeline Support.