Ok, to clarify, this is NOT a driver issue! I could not install linux mint 8 (Or any other version of linux or ubuntu for that matter) on my computer. It stopped at a black screen and froze. So I went to the BIOS and changed the default video card to my onboard 64mb, tried to install linux, and it worked. SO I rebooted, went back to BIOS, and had my nvidia 8400 gs chip run as the default video card. Well, while linux is booting, it will get as far as loading the logo, but then the screen goes black, and it freezes again. And this happens with and without drivers installed. Now here's the really weird thing- I think it's because my video card runs off of plain ol' PCI. I installed linux on 2 other computers using AGP and PCI-e, and there were no troubles. The AGP card was a geforce fx 1000, and the PCI-e card was a nvidia 9600 gt. So are there any ideas on how I can get my nvidia 8400 gs card running on linux mint 8?

What about...

Did you see this?

http://ubuntuforums.org/archive/index.php/t-615381.html

In addition, there may be a BIOS setting used to enable the installation of a PCI video card, that is, as well as one to disable the on-board chip. It seems that linux may well ignore the BIOS, but apparently not for all hardware in all cases. What is the make and model of your motherboard?

Have you tried to install the 8400GS PCI card in one of the other systems you mentioned and then to boot a live-cd to check that the card is in working order?

Otherwise, could you not boot with your on-board graphics enabled in the BIOS and the 8400GS installed but not enabled in the BIOS, and try to boot a live-cd. If and when you achieve a boot, and if and when the 8400GS is seen, then re-configure the graphics for the 8400GS, as a test.

I might recommend an openSUSE 11.2 KDE live-cd since openSUSE has good graphics recognition and is easy to re-configure, but some other linux may do as well.

My motherboard is an Intel 845GV chipset. I will try out my video card on another computer and see how that goes. It works just fine on windows, though.
I've installed linux doing that before, with my other video card plugged in. Then I get a window asking me to download proprietary drivers for Nvidia. I install it, and it tells me to reboot. I switch the cards via BIOS, but it only loads the logo for linux mint before I get a black screen. If I were to reconfigure the graphics, I would have to do it before I install the drivers.

Ok, it still didn't work :/ What should I do now?

Wow, I feel stupid.... I found your link But it's telling me to go to my hardware manager, and I can't find it. And I am also stuck using linux mint 7 right now because I lent my mint 8 cd to a friend so he could install it. I have to find out what pci bus my video card is on.

Ok, another problem- I did what the link told me to, and it ONLY ask me about my keyboard. Can someone give me baby steps with ALL possible explanations on what I'm doing please? I don't want to screw it up again.

This is what I was thinking...

What I meant was to use a live-cd other than one that is Ubuntu based and see if it finds both graphics cards/chips, then force it to choose the PCI to configure. This would tell you that the 8400GS will work with your motherboard and linux, and with the command "lsmod" tell you what modules are making it work, what the xorg.conf file looks like, etc.

As for practical help with the link I gave you, I'll get back to you if I find anything useful.