I'm no expert but I think it goes like this as far as what CM7 does with the 2gb's
/dev/store/cm-system [304 MB] <OS
/dev/store/cm-cache [200 MB]
/dev/store/cm-data [1.50 GB] < Apps(Android)
Pretty sure things like videos, photos, documents, etc are stored in the remaining space which is shared between CM7 an webOS. For example a video would be playable in both as long as it's compatable with the player.