o Ubuntu é derivado do Debian...
para instalar placas da nvidia basta abrir o terminal (xterm) ou (konsole) e digitar:
# apt-get install nvidia-glx
vai instalar esse pacote com as dependencias...

depois tem que editar o arquivo /etc/X11/Xorg.conf
modificando a linha Driver "nv" po "nvidia"

# ************************************************** ********************
# Graphics device section
# ************************************************** ********************

# Any number of graphics device sections may be present

Section "Device"
Identifier "VESA Framebuffer"
Driver "nv"
#VideoRam 4096
# Insert Clocks lines here if appropriate
EndSection

para:

************************************************** ********************
# Graphics device section
# ************************************************** ********************

# Any number of graphics device sections may be present

Section "Device"
Identifier "VESA Framebuffer"
Driver "nvidia"
#VideoRam 4096
# Insert Clocks lines here if appropriate
EndSection

reinicie o X:
ctrl + alt + backspace
$startx

se tudo ocorrer bem vai aparecer o logo da nvidia quandu o ambiente X for carregado.