DVD-Rs are the only ones you want for the superdrive. Many of them are labelled x1-x4 4.7Gb on the package, although some 'pure' x2 exist.
Retail price here in the UK can be as high as £5 each ($11, tax incl.), but I have recently bought some Verbatims on spindles of 25 at the very exciting price of £1 each ($2.30, tax included). This was by mail order and I am sure that there are similar outfits in Canada.
DVD+Rs are used one recordable DVD players (e.g. Philips), as well as some burners. DVD-RWs and DVD+RWs are for multiple sessions (like a VHS tape or a floppy) and are not yet supported by Apple drives.