Making a VGA cable for the Xbox is harder than normal because the video encoder does not output h/v-sync signals. Thus only Sync-On-Green supported monitors will work without adding a sync splitter circuit.
There are also cable select modes that configures the video encoder to output a specific group of signals (stereo/optical/component/vga/etc).
On the Xbox AV connector there are 3 pins that controls the type of signal the video encoder (Conexant/Focus) should output. By grounding a pin (low) you 'enable' the instruction and letting it be open (high) 'disabling' the instruction. There are 7 usable combinations shown in this chart by spillage.
For the VGA mode to work the Xbox has to output 480p (progressive) signal. To turn on progressive mode you need to attach a HD A/V (Component) cable (No S-Video or Composite output), turn on "Progressive Scan Mode (480p)" in the MS Dashboad, shutdown and attach the VGA cable.
But if you don't own a component ready TV you can join pin 11-12 and 13-14 on the VGA cable and it will be recgonized as a HD A/V cable. You can then blindly set the correct mode as follows. Remember to remove the loop on pin 11-12 and join pin 9-10 to enable VGA mode again.
Pause 1 Second
Pause 1 Second
Pause 1 Second
Restart your XBox, and you should be in 480p.
The problem on PAL consoles is that the HD A/V mode is completely disabled (imported HD Pack won't work at all) and you need to change the console region to NTSC (Video Mode Select, ConfigMagic, etc) before doing the abov procedure. You will of course get a black and white picture using regular S-Video and Composite cables on your TV but color on your VGA monitor.
To enable VGA mode you need to BIOS with VGA support There are not that may to choose from, but there are two older ones (XboxVGA and TATX Debug) and a newer (iND-BiOS) with the latest BIOS features. The VGA mode fixes the frequency and the green tint problem, but there are still games that somehow sets their own video modes and the green problem will still occur.
In VGA mode all three audio sources are available, stereo audio, SPDIF coaxial and optical audio.
Easy to wire, just connect the correct wires to the back of the vga connector, set the cable id (vga mode) on the back of the xbox connector (loops) and connect stereo audio cables to xbox connector.
This is a bit more tricky, you need to install a signal splitter, LM1881N, to extract sync from the green composite signal.
There are several different cables available with RGB input and can be modified to vga mod.
From best to worse (roughtly described):
Xbox-Scene VGA Cable tutorial [xbox-scene.com]
Xbox VGA HOWTO [xbox-linux.org]
Video Game Connectors [sourceforge.net]
Ind-bios Vga Support Thread [xbox-scene.com]
Quick Guide for High Def Output Thread [xbox-scene.com]
Microsoft Xbox A/V Pinout [Different numbering, use the system above] [gamesx.com]