IMHO, the best capacity planning tools that exist are trending/graphing solutions like Cacti, Munin or Ganglia. Monitor and graph everything you can: cpu usage, load, memory usage, swap, disk space, disk i/o, network traffic, etc. Graph not only system metrics, but also application ones (number of users, what are they doing, etc) and see how they relate to each other. This will tell you what you need to tune and how your platform is expected to grow in the future.
John Alspaw, VP of Operations at Etsy and ex-Flickr, has given a lot of talks on this subject. I'd recommend this great slides.
Hope this helps!