I'm trying to install Nvidia drivers for Debian, because I can get up to only 800 x 600 resolution, but the error message says "You appear to be running an X server; please exit X before installing." How do I get around this?

In order to install the NVidia drivers, X can't be running. If your computer boots into Linux and starts X automatically, here's what I'd do: edit the file /etc/inittab on the line that says "id:5:initdefault:". (note: yours might not say 5) Above that should be a table that says what number corresponds to which user mode. Replace the number on that line with the number corresponding to "multiuser mode" or "full multiuser mode". For example, on my system I would change '5' to '3'. DO NOT set it to 0 or 6! Now reboot. You will have to log in on the command prompt. Then you can install the drivers, and after that modify your X server's config file (usually /etc/X11/XFree86 or /etc/X11/xorg.conf) and change the video card driver line from "nv" to "nvidia". Now you can edit /etc/inittab and restore the original number to get your X login again after a reboot. If your computer doesn't automatically start X, there's probably a really easy way to stop the X server if you have root access. Then you can install the drivers and edit the config file.

To change runlevel without rebooting the system and editing inittab twice, just type:

init 3
(if you want to be absolutely certain nothing is running, use init 1 instead)

Thanks guys, that worked, but then it gave me another error, that it needed a kernel source. So I downloaded and installed the right kernel source, and it's giving me an error.

When I did init 1 that worked, but there was a message that said level 1 was not recommended for the installer because of problems with the kernel source. When I do init 3 though, I start in X.

try init 2, or once the login prompt appears press ctrl-alt-f1 to get a text console, log in as root, and type /etc/init.d/xdm stop