This topic is like 9 months old, I'm pretty sure they don't need help with this anymore..Burning speed depends on burning drive/chipset/hardware as well as the actual DVD, so unfortunately it might be trial and error. After a few coasters I've found that 2.4x works for Verbatim and 6x works for memorex consistently. Memorex seems a lot more picky, no other speeds worked for me.
Don't use memorex. Get Verbatim DVD+R DL Disks, they may be more expensive but its well worth it.