Just connect your iPhone to your Mac and use iPhoto to import your photos and videos. After they are imported, it will give you the option of deleting them from the phone. If you don't have a Mac, I'm sure there are photo managers on PCs that will import as well.
You can then selectively choose which photos, albums, events, etc. that you want to add back to your phone through iTunes. iTunes will talk to iPhoto to present your options for syncing photos. It all works really well.