You have not mentioned your graphics card. Is it onboard graphics or do you have a dedicated GPU?
1gb of ram is fine - 2 is better but 1 will run SU without a problem.
Your processor is not the most powerful and a better one could see some slight improvement.
The thing to remember though, if you are working with 3d trees and plants, even the most powerful of computers will have problems eventually. You need to be smart in the way you do things.
Use layers to control visibility of your trees.
Moving to the Hardware Forum.