You'll get wildly varying opinions here because everyone uses their computer differently. Some will say you need 8 GB or more because they need it to do heavy RAM intensive work. This is good advice if your usage is similar to theirs, but it may not be.
Others will disagree and say you only need around 2 GB because they only use their machine for word processing and light browsing and never use more than 1 GB. I only use about 200-300 MB out of 1.25 GB on my PowerBook at most, but I would never recommend 1.25 GB to the average user.
The only way to really know how much you need is to look at how much RAM you use doing normal tasks. It's not a perfect estimate because your computer may be wanting to use more RAM than it has, thus making it seem less needy, but it should be a good ballpark figure.
I think 4 GB is enough for a long time for your purposes.