Yo uso el controlador NV (el libre, el que se instala "de serie" con XORG), y puedo usar resoluciones mayores sin ningún problema. Ahora uso resolución de 1440x900. Creo que en vez de instalar el controlador privativo, antes deberías probar a reconfigurar las X.
No sé qué herramientas usa Slackware para configurar XORG, pero en todo caso puedes hacerlo a mano editando el archivo /etc/X11/xorg.conf. Mi XORG.CONF dice esto:
Cita: dandebian@dan:[~]$ cat /etc/X11/xorg.conf
# nvidia-xconfig: X configuration file generated by nvidia-xconfig
# nvidia-xconfig: version 1.0 (randy@juliet2) Fri Dec 30 21:53:37 PST 2005
# xorg.conf (Xorg X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the xorg.conf manual page.
# (Type "man xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following commands as root:
#
# cp /etc/X11/xorg.conf /etc/X11/xorg.conf.custom
# md5sum /etc/X11/xorg.conf >/var/lib/xfree86/xorg.conf.md5sum
# dpkg-reconfigure xserver-xorg
Section "ServerLayout"
Identifier "Default Layout"
Screen "Default Screen" 0 0
InputDevice "Generic Keyboard"
InputDevice "Configured Mouse"
EndSection
Section "Files"
# local font server
# if the local font server has problems, we can fall back on these
FontPath "unix/:7100"
FontPath "/usr/share/fonts/X11/misc"
FontPath "/usr/lib/X11/fonts/misc"
FontPath "/usr/share/fonts/X11/cyrillic"
FontPath "/usr/lib/X11/fonts/cyrillic"
FontPath "/usr/lib/X11/fonts/100dpi/:unscaled"
FontPath "/usr/lib/X11/fonts/75dpi/:unscaled"
FontPath "/usr/share/fonts/X11/Type1"
FontPath "/usr/lib/X11/fonts/Type1"
FontPath "/usr/share/fonts/X11/CID"
FontPath "/usr/lib/X11/fonts/CID"
FontPath "/usr/share/fonts/X11/100dpi"
FontPath "/usr/lib/X11/fonts/100dpi"
FontPath "/usr/share/fonts/X11/75dpi"
FontPath "/usr/lib/X11/fonts/75dpi"
EndSection
Section "Module"
Load "bitmap"
Load "dbe"
Load "ddc"
# Load "dri"
Load "extmod"
Load "freetype"
Load "glx"
Load "int10"
Load "record"
Load "type1"
Load "vbe"
EndSection
Section "InputDevice"
Identifier "Generic Keyboard"
Driver "keyboard"
Option "CoreKeyboard"
Option "XkbRules" "xorg"
Option "XkbModel" "pc105"
Option "XkbLayout" "es"
EndSection
Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
Option "CorePointer"
Option "Device" "/dev/psaux"
Option "Protocol" "IMPS/2"
Option "Emulate3Buttons" "false"
Option "ZAxisMapping" "4 5"
EndSection
Section "Monitor"
Identifier "Philips 107E4"
HorizSync 30.0 - 81.0
VertRefresh 56.0 - 75.0
Option "DPMS"
ModeLine "1440x900" 106.47 1440 1520 1672 1904 900 901 904 932 +HSync +VSync
EndSection
Section "Device"
Identifier "nVidia Geforce 4 MX 440"
Driver "nv"
Option "NvAGP" "3"
Option "NoLogo" "0"
Option "ConnectedMonitor" "CRT"
EndSection
Section "Screen"
Identifier "Default Screen"
Device "nVidia Geforce 4 MX 440"
Monitor "Philips 107E4"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
EndSection
Quizás te sirva como referencia para editar tu archivo XORG.CONF. En cualquier caso, si nos dices cuál es el modelo exacto de monitor que usas, a lo mejor existe alguien que ha publicado la configuración óptima,
como me pasó a mí.