10 gig is enough for most things. Just be sure that temporary files get deleted and if nesasary delete them from the recycling bin too.
About not burning on the fly, you are moving so much data so fast that you may be pushing the limits of your hardware. If data cannot be supplied fast enough then either the burn will fail, or the drive will have to slow down burning which is a problem. Dvds need to be burned at the proper speed and if you force the drive to slow down, it can cause a poor qulaity burn. I have burned dvd's on the fly before and they have came out and error scaned just fine, but in general, its not a relaible way of doing it. Some burns might go fine, others might complete but at the cost of burn quality, other burns may fail.