In a side by side it is very similiar. once you setup the new 2008 DB you can identify applications that connect to your DB's. you will need to work with the correct departments to ensure that they can be installed on a test machine with the ability to point them at the new instance. Once you have restored all DB's you will need to again script all logins and jobs to build on the new instance. Evaluate the any DTS packages independantly. Given the fact that the ugrade whizard does not work all that great it may be better to rebuild the DTS packages as SSIS packes manually. Once all applications are tested and packages built you can set you date for go live. If you keep and changes in cynch between the two environments in the mean time you should simply have to restore the DB's once last time and then repoint all live applications.
It will really depend on you applications and requirments how you go about the actual testing. It may be enough to simply connect and open a few screens or you may need to organize some sort of performance testing. There may also be specific scnarios you have to test. for example if you have any replication setup you would need to define that and then test those connections. If you have reporting services installed that is also another consideration.
the biggest advice I could give is use a method that is best for you to track everything. I write everything out on a white board. Every DB, Job, Replecation, DTS, application anything involved or affected. as they get tested and passed I place a green dot next to them. once everything has been cleared then I can proceed.
If only I could snap my figures and have all the correct indexes apear and the buffer clean and.... Start day dream here.