Did you use Netbook Installer to put OS X on? If you did, you should be able to just install Windows 7 as normal, using the XP partition, and then once that's done, boot back into the OS X dvd and run Netbook Installer and reinstall Chameleon. I don't know about the upgrading to Snow Leopard part because I've never done it. It might just be easier to back up all of your data, create a Snow Leopard DVD while you've still got OS X, and just reinstall the whole thing. It really only takes about an hour to get both up and running, it's not too bad.