Well if they are dual layer DVDs then you need to buy dual layer blanks...
You need to decrypt the DVDs too (since they are already on your hard drive, this is already done right?)
Good brand blanks are generally more compatible and last longer. Verbatim are a good brand but quite expensive. If you don't mind cheaper blanks then try out a few different brands and see what works best for you.
www.svp.co.uk is the best place to buy blanks
imo. They often do cheap 99p shipping weekends too. Its generally cheaper to buy blanks in 50s or 100s obviously...