It honestly depends on what you're doing. If you have a lot of programs open, the extra 2GB should make things a little less choppy, however for casual web browsing and just having the one or two games open I don't think it will make much of a difference.
7gig should allow you to multitask a LOT. You'll have to combine a lot of heavy stuff in order to need the other 2. Perhaps tell us what you normally do on your computer? f.e. gaming/skype/browsing the web
those are weird amounts of RAM.. Basically with RAM, if you're maxing out what you have, you'll see a difference with a an extra gig or two. If not, no difference.
Weird amounts of RAM as kill dank said.
From 7 to 9 gigs I dont think you would see any improvement honestly, especially if you are not doing anything intensive.