From: Max Fenstalker
(I was using a Nvidia 8400 at the time and SL worked fine)
So I updated my video card to a Nvidia 9400 GT.
So you know, just because a card is in a newer series doesn't mean it's a better card. With nVidia's 4-digit numbering convention, the second digit is typically the most important. The first digit denotes the manufacturing series, and the second tells the card's performance level within the series.
The only significant difference between the 8 and 9 series is power consumption. nVidia chanced their manufacturing process with the 9 series, allowing them to create cards that offered the same performance levels and lower power usage levels. Aside from some minor bells and whistles added to a few of the various models, that's really it.
I hate to tell you this, but if you look at the actual performance specs, it's arguable that you actually downgraded, rather than upgraded. On many crucial performance specs, the 8400 ranks somewhere in between a 9400 and a 9500. The 9500 is considered to be the successor to the 8400. The 9400 is a small step down in performance.
So, you upDATED, but you didn't necessarily upGRADE, if you follow my meaning. A real upgrade would have meant a higher second digit, regardless of series. Newer doesn't always mean better, in and of itself. You have to look at the actual specs when making decisions.
From: Max Fenstalker
I checked the SL system requirements page and it says "NVIDIA GeForce 6600 or better"
And I do have better
It's debatable whether you have "better". You have "newer", certainly, but not necessarily "better". Ignore for a moment the fact that the 6600 has an older interface (AGP instead of modern PCIE), so it's not going to be usable in most modern machines, the actual performance specs of the 6600 are about the same as those of the 8400 and in most areas, and are actually better than 9400 in some.
Again, the second digit is what's most important with nVidia cards. The first doesn't tell you much beyond when the card was designed. For performance, the second digit is your first indicator. After that, look at the specs, and make an educated purchasing decision.
I hate to make anyone feel bad about a purchase they've already made, but the truth is what it is.
From: Max Fenstalker
but the page doesn't specifically list the Nvidia 9 series
So does SL have a problem running the Nvidia 9 series?
SL has no particular problem with the 9 series, but it does have problems with low end cards of all types. And I hate to break it to you, but anything with a 4 in the second digit is low end. I wouldn't go with anything below a x6xx myself.
That said, it should work, just not incredibly well.
My guess is you've got a driver problem, especially given the fact that you said you tried to use Add/Remove Programs to upgrade. Don't EVER do that. Windows is not capable of fully removing old drivers on its own, let alone installing new ones. This is where a lot of people run into trouble. You need a good driver removal program. I recommend DriverCleaner. I've never used Driver Sweeper, but I'd imagine it probably would do the job too.
Here's the procedure I recommend you always follow when updating drivers:
1. Download the latest driver from the nVidia website. Don't install it yet; just download it.
2. If you don't already own DriverCleaner, go to drivercleaner.net, and purchase it for $10. It's well worth it. It's the only way I know of to ensure a completely clean uninstallation of an existing driver, short of removing all files and registry entries by hand, which is not a good idea to try.
3. Restart Windows in Safe Mode. To do this, tap F8 as your computer is first booting up, to bring up the Boot Manager, and then pick Safe Mode from the list.
4. Run DriverCleaner now, and remove your existing nVidia graphics driver.
5. Restart normally.
6. Now install the driver you downloaded in Step 1.
With any luck, SL should work now. If not, then my next guess would be something is physically wrong with your hardware. Perhaps the new card isn't seated properly. Try removing it and putting it back in place. It's also possible it's overheating, or that it's simply a bad card. If you can't get it to work, call nVidia, and have them send you out a warranty replacement. If the new one has the same problem, then you may have damaged your motherboard when you swapped out the cards. Hope that that's not the case, because replacing a motherboard is no small task.