Changing from 7 to 9 speed is not so easy, they use different freehubs and the wheel might need re-dishing.
To be honest, there's very little real world advantage in having 3 x 9 gears over 3 x 7 gears. I think it has more to do with marketing than anything else. I have one bicycle with 21 gears and another with 27 and I never notice the addition of, or miss those extra 6 gears when riding either of the two bicycles.
Here's everything, and probably more, that you wanted to know about cassettes, freehubs, etc. :
http://sheldonbrown.com/speeds.html
http://sheldonbrown.com/k7.html
In addition, you're unlikely to notice much difference in performance going from Tourney to Acera. In my experience, there isn't a big a difference in performance and functionality when moving up or down groupsets as the price might indicate. What you do get with the more expensive components is generally a nicer finish, and lighter weight. Also, the cheapest, bottom of the range components tend to use a lot of steel, and lower quality, bolts etc. which aren't stainless, so corrosion is more of an issue.
What is your aim with the upgrade?