Changing the advanced relevance settings for Search Server 2008 and SharePoint Server 2007 (such as the global ranking parameters, property weights and property length normalization) isn't something that you'll want to do without a lot of thought, planning and testing.
And in the vast majority of cases you'll never need to go near the settings, but in certain specialized deployment scenarios it might be something you want to consider.
This MSDN article and TechNet White Paper give you a very detailed review of the relevance knobs and wheels you can turn along with how to structure and plan your testing to see if you're achieving your desired end result.
To help you on your way, Christopher Even (http://www.sharepointsearch.com) has written and shared on CodePlex a small utility that allows you to change all of the advanced relevance settings quickly and easily.
The tool is here and is part of the Search Community Toolkit.
This tool comes with a relevance health warning: Read the MSDN and TechNet content before you start, use the tool with care, make a note of all of the values that you change before you change them and ensure you're consistent and diligent in your relevance testing after you make any changes!
There's nothing that the tool does that can't be done though the search object model, it just saves you having to write code to tune the various advanced parameters.
Richard Riley Senior Technical Product Manager Microsoft Corp