Making an SCCM query is quite easy, and for Vista, you can start with the query for XP, copy and modify slightly to create a Vista query. These steps are documented in detail on the ConfigMgr forums on TechNet Social. See the following.

http://social.technet.microsoft.com/Forums/en-US/configmgrsetup/thread/965dc668-31da-4750-addc-334c157de50c

The same would be true for other Operating Systems as you begin to manage them with ConfigMgr 2007.

Being that you are in a lab environment and testing RC OS's, there is functionality managing Win7 with SCCM R2. (gotta test this stuff somehow right!). All my Win7 systems are managed with Lab SCCMR2 and you get the core management like, updates, software adverts, App-V ect. Yes, need to wait till SP2 bits for any OS deployment testing.

For the collection, like the other post says, open up properties for All XP systems collection>Double click the query rule>Click Edit query statement>Click Show Query Lanuage and copy the highlighted query. Cancel out of the wizard and right click the collection root>New Collection>type All Windows 7 Systems>Next, click yellow query rule button, re-type name again>Click Show query lanuage and paste previous query. Then just modify the version to 6.1. OK out of wizard and refresh new collection.

You have totally solved an issue in my lab testing of collections around Windows 7....well done!