Please read http://blogs.msdn.com/sudhir/archive/2007/08/03/upgrade-tfs-2005-with-wss2-0-to-tfs-2008-orcas-with-existing-wss3-0-farm.aspx if you want to upgrade to existing WSS3.0 farm.
This topic is very similar to the previous topic(http://blogs.msdn.com/sudhir/archive/2007/05/31/upgrade-2005-with-wss2-0-to-orcas-wss3-0.aspx). The primary difference is how will you move to WSS3.0 on remote machine.
The steps remain same. In short
/SharepointUnc:\\wssserver\sites
Sudhir has been blogging up a storm lately. He has a series of posts describing what we've done to improve