My best advice would be to stay well clear of the "off" brand DVD burners. I've helped way too many people having issues with them. Quality counts for quite a bit when it comes to DVD burners.
My highest recommendations go to Pioneer, Plextor and Lite-On for the best quality and performance. Sony's seem to be decent enough but I don't care for them personally.
The dual layer question boils down to how bad you need this feature and how much you'll really use it. The main reason to get a DL burner is the ability to back up DVD-9 movies without having to remove features or compress the film and in order to do that you'll need software that supports DL burning such as Nero 6, Elby's CloneDVD 2, DVD Decrypter or BlindWrite 5. Another point to keep in mind is that there isn't any DL rewritable media and the cheapest DL media available is $9 a pop so if somwthing goes wrong you just flushed nine bucks down the toilet. And to reiterate Praetor's observation, Memorex isn't good DVD media because they buy from whoever is cheapest at the moment and slap their name on it so you don't know if you're buying decent media or getting junk - they've been known to use both.
The absolute best values for DVD burners right now are both Pioneers - the 107 is an outstanding SL 8X burner that goes for around $80 now and the 108 is the new DL burner they just introduced.
In the DVD burning game the word to remember is quality. A quality drive is the first key and quality media is just as important.