I've been coming across this type of postings alot. For some reason, people think that RAM requirement will grow with time. I know Bill Gates could be evil but I don't think his company are getting incomes from the sales of RAM. So I doubt very much that Windows 7 will start eating up your RAM over time.
One of my clients is a global I.T. consulting company. They provided a PC for me and I've been using the same PC for the last 4 years with one 1GB of RAM, the CPU is a Intel dual core. The PC is still running great and I am planning to use the same PC with no hardware upgrade for the next 5 years. If my system gets upgraded, the request will not be coming from me because I don't need it.
Is it possible that you will need 8GB of RAM in the future? Sure that's possible but it's not because of time. Your RAM requirement is based on the applications running on your computer and the number of applications you would want to run at the same time. Any PC could run one applications nicely but a PC with more RAM could handle multiple applications nicely.