bitbytebit: It looks standard. 15.1 - 16.8 kHz horizontal, 47 - 63 Hz vertical. Resolution 560x240 lines.
Here is the output from "groovymame -verbose -md 4 dkong"
Parsing mame.ini
Parsing mame.ini
Monitor: generic Orientation: horizontal
MonitorLimits 15725.00-15750.00,50.00-60.00,2.000,4.700,8.000,0.064,0.192,1.024,0,0,288.0,448
# dkong [11] 400x256@57.07 15.7500Khz
ModeLine          "400x256x57.07" 8.190000 400 416 456 520 256 257 260 276 -HSync -VSync
SwitchRes [dkong] (1) vertical (256x224@60.61)->(400x256@60.61)->(400x256@57.07)
# dkong 400x256@57.07 15.7500Khz
   ModeLine          "400x256x57.07" 8.190000 400 416 456 520 256 257 260 276 -HSync -VSync
ADD VGA-0:    ModeLine          "400x256x57.07" 8.190000 400 416 456 520 256 257 260 276 -HSync -VSync
SwitchRes Running 'xrandr --newmode      "400x256x57.07" 8.190000 400 416 456 520 256 257 260 276 -HSync -VSync'
SwitchRes Running 'xrandr --addmode VGA-0 "400x256x57.07"'
Setting SDL audiodriver 'dsp' ...
Build version:      0.141u3 (Mar  5 2011)
Build architecure:  SDLMAME_ARCH= 
Build defines 1:    SDLMAME_UNIX=1 SDLMAME_LINUX=1 
Build defines 1:    LSB_FIRST=1 DISTRO=generic SYNC_IMPLEMENTATION=tc 
SDL/OpenGL defines: SDL_COMPILEDVERSION=1214 USE_OPENGL=1 USE_DISPATCH_GL=1 
Compiler defines A: __GNUC__=4 __GNUC_MINOR__=4 __GNUC_PATCHLEVEL__=4 __VERSION__="4.4.4" 
Compiler defines B: __unix__=1 __i386__=1 
Compiler defines C: __USE_FORTIFY_LEVEL=0 
SDL Device Driver     : x11
SDL Monitor Dimensions: 648 x 480
Enter sdlwindow_init
Using SDL single-window OpenGL driver (SDL 1.2)
Leave sdlwindow_init
 648x 480 -> 0.001541
 400x 256 -> 2.000000
Loaded opengl shared library: <default>
OpenGL: Tungsten Graphics, Inc.
OpenGL: Mesa DRI R100 (RV200 5157) 20090101 x86/MMX/SSE2 TCL DRI2
OpenGL: 1.3 Mesa 7.9.1
OpenGL: texture rectangle supported
OpenGL: non-power-of-2 textures supported (old method)
OpenGL: vertex buffer supported
OpenGL: pixel buffers not supported
OpenGL: framebuffer object supported
OpenGL: GLSL not supported
OpenGL: max texture size 2048 x 2048
Keyboard: Start initialization
Input: Adding Kbd #1: System keyboard
Keyboard: Registered System keyboard
Keyboard: End initialization
Mouse: Start initialization
Input: Adding Mouse #1: System mouse
Mouse: Registered System mouse
Mouse: End initialization
Joystick: Start initialization
Joystick: End initialization
Audio: Start initialization
Audio: Driver is dsp
Audio: frequency: 48000, channels: 2, samples: 512
sdl_create_buffers: creating stream buffer of 57344 bytes
Audio: End initialization
ouput: unable to open output notifier file /tmp/sdlmame_out
Input: Changing default joystick map = s8.4s8.44s8.4445
  s8888888s
  4s88888s6
  44s888s66
  444555666
  444555666
  444555666
  44s222s66
  4s22222s6
  s2222222s
Input: Changing default joystick map = s8.4s8.44s8.4445
  s8888888s
  4s88888s6
  44s888s66
  444555666
  444555666
  444555666
  44s222s66
  4s22222s6
  s2222222s
Starting Driver Device 'root'
  (missing dependencies; rescheduling)
Starting Z80 'maincpu'
Starting DMA8257 'dma8257'
Starting Video Screen 'screen'
Starting 8 bit latch 'ls175.3d'
Starting 8 bit latch 'ls259.6h'
Starting 8 bit latch 'virtual_p2'
Starting MB8884 'soundcpu'
Starting Speaker 'mono'
  (missing dependencies; rescheduling)
Starting DISCRETE 'discrete'
Starting Driver Device 'root'
  (missing dependencies; rescheduling)
Starting Speaker 'mono'
Starting Driver Device 'root'
 648x 480 -> 0.001541
 400x 256 -> 2.000000
OpenGL: VBO supported
OpenGL: FBO supported
OpenGL: using vid filter: 0
GL texture: copy 1, shader 0, dynamic 0, 256x224 256x224 [PALETTE16, Equal: 0, Palette: 1,
            scale 1x1, border 0, pitch 384,256/2048], colors: 521, bytes/pix 4
GL texture: copy 1, shader 0, dynamic 0, 256x224 256x224 [PALETTE16, Equal: 0, Palette: 1,
            scale 1x1, border 0, pitch 384,256/2048], colors: 521, bytes/pix 4
sdl_kill: closing audio
Joystick: Start deinitialization
Joystick: End deinitialization
Enter sdlwindow_exit
Leave sdlwindow_exit
REMOVE VGA-0:    ModeLine          "400x256x57.07" 8.190000 400 416 456 520 256 257 260 276 -HSync -VSync
SwitchRes Running 'xrandr --delmode VGA-0 "400x256x57.07"'
SwitchRes Running 'xrandr --rmmode "400x256x57.07"'
Here is my xorg.conf
# Config for generic Monitor and ati Video Card
Section   "Monitor"
   Identifier   "VGA-0"
   VendorName   "Unknown"
   ModelName   "Unknown"
   HorizSync   15-50
   VertRefresh   40-80
   Option      "DPMS"   "False"
   Option      "DefaultModes"   "False"
   UseModes        "ArcadeModes"
EndSection
Section "Device"
   Identifier   "Card0"
   VendorName   "Unknown"
   BoardName   "Unknown"
   Driver      "ati"
   BusID      "PCI:01:00:0"
   Option      "ModeDebug"   "true"
   Option      "monitor-VGA-0"   "VGA-0"
EndSection
Section "Screen"
        Identifier "Screen0"
        Device     "Card0"
        Monitor    "VGA-0"
        DefaultDepth    24
        SubSection "Display"
                Viewport   0 0
                Depth     8
        EndSubSection
        SubSection "Display"
                Viewport   0 0
                Depth     16
        EndSubSection
        SubSection "Display"
                Viewport   0 0
                Depth     24
        EndSubSection
   EndSection
Section "Modes"
   Identifier      "ArcadeModes"
#  648x480@60.00 15.7500Khz
   ModeLine          "648x480x60.00" 13.230000 648 672 736 840 480 484 490 525 -HSync -VSync interlace
EndSection