What are the general must have addons (that are not airplane specific) for FS9? I want to make it look really good.
Get Boston Logan airport free from www.flytampa.com it's a beauty.
Back up your texture folder and try this, see how you like it.
I used it and you can see all the downloads it's had.
I would start with Flight1's:
Ground Environment Pro
Ultimate Terrain USA / Europe
Yeah, get FE Pro and GE Pro and Ultimate Water, that will improve your whole environment by a lot, then start getting addons that suit your flying area
Make sure to update your graphics card drivers, its a simple way to boost performance cheap!