If you are currently using your onboad VGA chip then certainly a Geforce 8500 will be an improvement. But it is a really old card that doesn't support DirectX 10 if I remember correctly. I can't even find it on sale anymore. Unless you can get it for very little money, I would probably look for a more modern budget card.
There are various threads on what addons to use for improved terrain etc., but beware, you will lose some fps. I'd probably max out FS9 first, then add addons.
Intel Core 2 Duo 6300, 8 GB DDR2-800, ATI Radeon HD3870, Vista 64