UPDATE: Intrepid has zero problems with this video card, at least from my experience. The nvidia-glx-new package works just fine.
Here's a little how-to guide for those of you in my situation (also a how-to guide for me, since whenever Ubuntu does kernel upgrades it wipes out my drivers and my memory is usually pretty foggy). I have an Nvidia Geforce 8800 GT and here is my guide to getting it working under Ubuntu. Unfortunately, as of the time of this writing, the card does not work out of the box with Ubuntu - surprising for Nvidia cards, I know. The default nv driver "works" in that you can see things, but running at 640x480 on a 20" widescreen is not my definition of "working".
EDIT: This doesn't work with Hardy yet (as of May 12, 2008). You can install the nvidia-glx-new package which has the drivers, but there is still a glitch with the 8000 series where your title bars won't show up (they'll be there, but are invisible) if you have Compiz Fusion on. I'll put more information about this as soon as I find a fix for it.
EDIT2: One of my updates fixed the title bars thing. Things work perfectly now. Firefox still crashes.
EDIT3: Forgot, you need a few packages before you get going. Install the build-essential and your kernel header packages or the Nvidia installer will not be able to compile a kernel interface for the driver. To do this, type:
sudo apt-get install build-essential linux-headers-`uname-r`and hit Enter. Remember that those are backquotes, which are in the top-left of your keyboard under the Esc key.
Before you start, you should be reading this on a separate computer, or memorizing it. You'll need to shut down X for this - don't worry, I'll hold your hand. Since you'll be shutting down the X server, anything running in X will be shut down too. So save anything important now.
First, you need to get the Nvidia drivers. If you hate proprietary things, then go buy an older video card or write your own drivers, because the open-source drivers don't work. I tried. You can get the drivers on Nvidia's website, just pick the model you have and the OS (I'm assuming Linux, so choose either Linux 32-bit or 64-bit depending on what you have). Save this file to your desktop (or wherever you want, if you know what you're doing).
Now you need to kill X. To do this, press Ctrl+Alt+F1 to switch to the console. Type your username and password there to log in. Now, type:
sudo /etc/init.d/gdm stopand press enter. You'll be asked for your password again, so type it in. This will shut down the X server. Hopefully you took my advice and saved anything you wanted to keep.
Now you're in the console with X shut down. Congrats, you've risen to a higher level of geek. Now you need to install the driver. Type:
cd Desktopand press enter. This puts you in your Desktop folder, where the Nvidia driver was saved - if you didn't save it to your desktop, I'm assuming you know how to use cd so go to whatever folder you saved the file to. Type
lsand press enter to see the files there. You should see one that starts with NVIDIA-Linux somewhere, followed by your architecture (x86 for 32-bit, x86_64 for 64-bit) and the version number of the driver. If you have lots of things on your desktop, you can use Shift+PageUp and Shift+PageDown to scroll up and down through the listing.
Now to install the driver:
sudo sh NVIDIAand press Tab. This should automatically fill in the filename, if it doesn't then just type out the whole filename that you saw after you used ls. Now press enter.
This will start up the NVIDIA installer, and if everything went well, it should start going. You have to use the arrow keys and enter to do everything, but trust me, it's not that hard. Usually you just need to do "Next" or "Accept" or "Yes" all the time and it should all work - unless you've customized your xorg.conf file, but I'm assuming that if you can do that you're probably not needing my help anymore.
After the installer has done all its magic, you should be ready to start up again. Type:
sudo shutdown -r nowand press enter to restart your computer. When it boots up again, you should have a fancy new graphics card installed and will be able to set your screen resolution, play cool games (some DO exist for Linux, I assure you) and all that jazz. Enjoy!
PS: I'm hoping Nvidia makes it easier for Linux users to install drivers (ie. not having to use the command-line) but for now, you'll have to do it this way.