Similar to what Sligachan14 said, there is 'packet writing' with Roxio - it's called Direct CD. I think this is now bundled as part of Windows XP? Personally I preferred the old method of formatting the CDRW first (even though it took a while) and then just used the CDRW in the same way that you would use a floppy. Now you don't need to format (it does it 'on the fly') but you need to wait a while when you eject the disc. I haven't used them for a while now that we have memory sticks for transferring from machine to machine, and I use CDR or DVR for backups.
Personally, I would have a folder on the hard drive and keep adding photos to it until I reach 700MB and then archive to CDR. (Maybe using CDRW as a temporary archive (belt and braces and all that).