compiz --replace: Another window manager is already running on screen: 0

[abalone]

Compiz used to work on previous installations - now I'm told this:

me@gutsy:~$ compiz --replace
Checking for Xgl: not present.
Detected PCI ID for VGA: 01:00.0 0300: 10de:02e1 (rev a2) (prog-if 00 [VGA])
Checking for texture_from_pixmap: present.
Checking for non power of two support: present.
Checking for Composite extension: present.
Comparing resolution (3200x1200) to maximum 3D texture size (4096): Passed.
Checking for nVidia: present.
Checking for FBConfig: present.
Checking for Xgl: not present.
Starting emerald
/usr/bin/compiz.real (core) - Error: Another window manager is already running on screen: 0
/usr/bin/compiz.real (core) - Fatal: No manageable screens found on display :0.0
Window manager warning: Failed to load theme "Clearlooks": Failed to find a valid file for theme Clearlooks

(Then I run kwin --replace to wrest my "KDE experience" back out of the hands of metacity)

xorg.conf (minus mouse, keyboard, etc. ...):

Section "ServerLayout"
  Identifier  "Default Layout"
  Screen   0 "Screen0" 0 0
  InputDevice  "Generic Keyboard"
  InputDevice  "Configured Mouse"
EndSection

Section "Files"
  FontPath    "/var/lib/defoma/fontconfig.d/B"
  FontPath    "/usr/share/fonts/truetype/ttf-bitstream-vera"
  FontPath    "/var/lib/defoma/fontconfig.d/N"
  FontPath    "/usr/share/fonts/type1/gsfonts"
EndSection

Section "Module"
  Load     "glx"
  Load     "v4l"
  Load     "freetype"
EndSection

Section "Monitor"
  Identifier  "Monitor0"
  VendorName  "Unknown"
  ModelName   "AOC Spectrum7K"
EndSection

Section "Device"
  Identifier  "Videocard0"
  Driver    "nvidia"
  BoardName   "nv"
  Screen     0
EndSection

Section "Screen"
  Identifier  "Screen0"
  Device    "Videocard0"
  Monitor    "Monitor0"
  DefaultDepth  24
  Option    "TwinView" "1"
  Option    "metamodes" "CRT-0: 1600x1200 +0+0, CRT-1: 1600x1200 +1600+0; CRT-0: NULL, CRT-1: 1280x1024; CRT-0: NULL, CRT-1: 1280x960; CRT-0: NULL, CRT-1: 1024x768; CRT-0: NULL, CRT-1: 800x600; CRT-0: NULL, CRT-1: 640x480"
  Option    "AddARGBGLXVisuals" "True"
  SubSection  "Display"
  Depth   24
  Modes   "nvidia-auto-select"
  EndSubSection
EndSection

Section "Extensions"
  Option    "Composite" "Enable"
# makes no difference, though
EndSection

Can't think of much else to post in the way of information...

Some version numbers from copied Synaptic (I'm sure that was the least elegant way):

compiz-core 1:0.6.2+git20071119-0ubuntu1~gutsy1
compiz-fusion-plugins-extra 0.6.0+git20071121-0ubuntu1~gutsy1
compiz-fusion-plugins-main 0.5.2+git20070928-0ubuntu2
compiz-kde 1:0.6.2+git20071119-0ubuntu1~gutsy1
compiz-plugins 1:0.6.2+git20071119-0ubuntu1~gutsy1
libdecoration0 1:0.6.2+git20071119-0ubuntu1~gutsy1
emerald 0.3git20070717-0ubuntu1
nvidia-glx-new 1:100.14.23+2.6.22-14
linux-image-2.6.22-14-generic 2.6.22-14.52
linux-image-generic 2.6.22.14.21
linux-ubuntu-modlues-2.6.22.-14-generic 2.6.22-14.38
linux-restricted-modules-2.6.22-14-generic 2.6.22.4-15.10
linux-restricted-modules-common 2.6.22.4-14.10
linux-restricted-modules-generic 2.6.22.14.21
nvidia-kernel-2.6.22-14-generic 100.14.23-0ubuntu3+2.6.22-14.52
nvidia-new-kernel-source 1:100.14.23+2.6.22-14
nvidia-kernel-common 20051028+1ubuntu7
xorg 1:7.2-5ubuntu13
xserver-xorg 1:7.2-5ubuntu13
xserver-xorg-core 2:1.3.0.0.dfsg-12ubuntu8.3

This is Mint 4.0 KDE CE, Nvidia drivers installed through envy (getting nvidia-glx-new through apt-get didn't seem to work this time).