I agree with shldr2thewheel. When it comes to RAM, there's no such thing as too much. My MacBook2,1 has 4GB installed (3GB usable). I ran with 2GB for years, but when I started using Parallels Desktop regularly for localization work, the extra GB came in VERY handy.
At the end of the day, it's mostly about the applications you use. Memory is cheap. The best advice I can give is to always put in as much as you can.