Matt makes some really good points, but the key here is that SEM/SEO these days has to be about more than just driving traffic - it has to be about how that traffic converts, and if you're using analytics software that can't analyse conversions then I'm afraid you're way behind the curve.
In particular there are two new and free packages of second-generation web analytics tools that focus on conversion tracking - Google Analytics (http://www.google.com/analytics
) and ClickTracks Appetizer (http://www.clicktracks.com/products/appetizer/)
. The GA product is more fully-featured, but both are focused on conversion tracking as the ultimate KPI.
vangogh - you're spot on with your post - trends are far more useful metrics than absolute numbers.