My stock answer to nearly all website measurement queries is to recommend Google Analytics.
Its easy to install; gives great reports; can plot a sales funnel and so much more. Unbelievably its free!
What is even more unbelievable is the amazing array of Analytics tweaks and custom tracking ‘extensions’.
For example, Analytics doesn’t report a couple of key pieces of info ‘out of the box’ ie
• where do visitors go when they leave your site?
• what has been downloaded from my site?
To answer these questions you would normally add special Analytics scripts to every link etc Tedious!
Good Web Practices has a nifty script that dynamically captures site exits and PDF and other downloads in Analytics – well worth a look.