J'ai eu un peu de mal, mais c'est passé. J'ai reconfiguré mon bi-écran, et j'ai pu lancer Beryl. Je me suis inspiré de ma configuration existance, que j'ai légèrement modifiée, sinon, pas d'effet 3D :( D'ailleurs c'est grâce à un topic du forum du club Mandriva que j'ai obtenu les petites options qui font toute la différence pour Beryl.

Voici donc Ze fichier xorg.conf :

Section "Files"
    # font server independent of the X server to render fonts.
    FontPath "unix/:-1"
    
    # minimal fonts to allow X to run without xfs
    FontPath "/usr/share/fonts/misc:unscaled"
EndSection

Section "Extensions"
    Option "Composite" "Enable"
    Option "RENDER" "Enable"
EndSection

Section "ServerFlags"
    #DontZap # disable <Crtl><Alt><BS> (server abort)
    AllowMouseOpenFail # allows the server to start up even if the mouse does not work
    #DontZoom # disable <Crtl><Alt><KP_+>/<KP_-> (resolution switching)
EndSection

Section "Module"
    Load "dbe" # Double-Buffering Extension
    Load "v4l" # Video for Linux
    Load "extmod"
    Load "type1"
    Load "freetype"
    Load "/usr/lib/xorg/modules/extensions/nvidia/libglx.so"
    #Load "glx" # 3D layer
EndSection

Section "InputDevice"
    Identifier "Keyboard1"
    Driver "kbd"
    Option "XkbModel" "microsoftmult"
    Option "XkbLayout" "fr"
    Option "XkbOptions" "compose:rwin"
EndSection

Section "InputDevice"
    Identifier "Mouse1"
    Driver "mouse"
    Option "Protocol" "ExplorerPS/2"
    Option "Device" "/dev/mouse"
EndSection

Section "Monitor"
    Identifier "monitor1"
    VendorName "BenQ"
    ModelName "BenQ FP71E"
    HorizSync 31-83.0
    VertRefresh 56.0-76.0
    
    # TV fullscreen mode or DVD fullscreen output.
    # 768x576 @ 79 Hz, 50 kHz hsync
    ModeLine "768x576"     50.00  768  832  846 1000   576  590  595  630
    
    # 768x576 @ 100 Hz, 61.6 kHz hsync
    ModeLine "768x576"     63.07  768  800  960 1024   576  578  590  616
EndSection

Section "Device"
    Identifier "device1"
    VendorName "nVidia Corp."
    BoardName "NVIDIA GeForce FX (generic)"
    Driver "nvidia"
    Option "DPMS"
    Option "RenderAccel" "True"
    Option "DisableGLXRootClipping" "true"
    Option "BackStoring" "True"
    Option "MetaModes" "DFP-0: 1280x1024 +0+0, CRT-0: 1280x1024 +1280+0 ; 1024x768, 1024x768 ; 800x600, 800x600 ; 640x480, 640x480 ;"
    Option "SecondMonitorVertRefresh" "50-120"
    Option "ConnectedMonitor" "DFP-0, CRT-0"
    Option "TwinViewOrientation" "CRT-0 RightOf DFP-0"
    Option "AllowGLXWithComposite" "true"
    Option "TwinView" "true"
    Option "SecondMonitorHorizSync" "30-70"
    Option "IgnoreEDID" "0"
EndSection

Section "Screen"
    Identifier "screen1"
    Device "device1"
    Monitor "monitor1"
    DefaultColorDepth 24
    
    Subsection "Display"
        Depth 8
        Modes "1280x1024" "1152x864" "1024x768" "832x624" "800x600" "640x480" "480x360" "320x240"
    EndSubsection
    
    Subsection "Display"
        Depth 15
        Modes "1280x1024" "1152x864" "1024x768" "832x624" "800x600" "640x480" "480x360" "320x240"
    EndSubsection
    
    Subsection "Display"
        Depth 16
        Modes "1280x1024" "1152x864" "1024x768" "832x624" "800x600" "640x480" "480x360" "320x240"
    EndSubsection
    
    Subsection "Display"
        Depth 24
        #Modes "1280x1024" "1152x864" "1024x768" "832x624" "800x600" "640x480" "480x360" "320x240"
        Virtual 2560 1024
    EndSubsection
    Option "AddARGBGLXVisuals" "True" # Necessaire pour Beryl
EndSection

Section "ServerLayout"
    Identifier "layout1"
    InputDevice "Keyboard1" "CoreKeyboard"
    InputDevice "Mouse1" "CorePointer"
    Screen "screen1"
EndSection

Pour information, je possède une carte vidéo Nvidia Geforce FX 5700 VE, j'ai un écran plat BenQ et un cathodique Packard Bell. Le cathodique est à droite, le plat à gauche.