For the average use of using Office programs, email, chat, and browsing the net, the Macbook will easily last you several years. If you only have 1GB of RAM right now, you might be looking at an upgrade later on, especially with future OS X revisions, such as 10.6, and possibly 10.7 as Apple has an update schedule of 18 to 24 months.
You might also end up getting an external hard drive as you begin to accumulate stuff like documents, videos, and music.