Just read a good article on installing SPS on a server with WSS already installed.

I found this article referenced over on Ian's site http://www.wssdemo.com.

Article is here:

http://www.microsoft.com/technet/prodtechnol/office/sps2003/deploy/inwsssps.mspx

I am working with a number of organisations right now who are installing and getting a WSS infrastucture up and running with a view to adding SPS in the next few months ... so this article contains some great info on what you might need to concider.