I'm running a 64bit Ubuntu 10.04 here. Here's what I found out:
lspci will give you a list of found devices. Usually the video card is listed as "VGA controller" or somesuch, on my system it says:
01:00.0 VGA compatible controller: nVidia Corporation G92 [GeForce 9800 GTX] (rev a2)
If you meant X.org's video driver, see /etc/X11/xorg.conf and look for Driver in Section "Device" (there might be multiple):
Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce 9800 GTX/9800 GTX+"
EndSection
Section "Device"
Identifier "Device1"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce 9800 GTX/9800 GTX+"
BusID "PCI:1:0:0"
Screen 1
EndSection
If you need to find out the specific location of driver-file and version X.org's using, try cat /var/log/Xorg.0.log and look for line which says something like LoadModule "'Driver in the Device-section of xorg.conf', in my case:
(II) LoadModule: "nvidia"
(II) Loading /usr/lib/xorg/extra-modules/nvidia_drv.so
(II) Module nvidia: vendor="NVIDIA Corporation"
compiled for 4.0.2, module version = 1.0.0
Module class: X.Org Video Driver
(II) NVIDIA dlloader X Driver 195.36.24 Thu Apr 22 19:18:54 PDT 2010
(II) NVIDIA Unified Driver for all Supported NVIDIA GPUs
(II) Primary Device is: PCI 01@00:00:0