Anything over 6xxx for the best driver...
Nvidia is the standard. Installation is, in most cases, easier and more problem free, and performance is better than the alternatives. The 8400GS seems to come in two versions. It is though a question of what you can get for a good price.
I would google with the graphics card make and model or the nvidia chip and linux nvidia driver to see how the combination works for others. Make sure that they can run the new 185.18.14 driver version without trouble.
I have a reserve machine put togther from the scrap heap with a passively cooled 6200 chip card that works fine. You might try to get a used card, perhaps on-line from Blocket, Gule Tidningen, or a similar site, or on-line from a gamer or overclocker web site. Below is one from DK. Those people are always building and re-building, and sell the extras, both the newer and older.