I too had a problem with burning dl disks. I have 2 observations.
1. The obvious question is: Are you sure that the standard Dell Optiarc dvd+rw 7580S drive is a dual layer burner? It might be only a single layer burner.
2. Assuming that the drive is a true dl burner, the problem is more than likely your disks. I have 2 dual layer burners, and one of them (the older one) completely refuses to burn a memorex disk. The newer burner will usually burn a dual layer disk, but it is really hit and miss with that drive as well.
I have read several forum posts where all dual layer disks are problematic with the exception of Verbatim. I found a 50 disk cakebox of Verbatim disks on Amazon for $49.95 ($1 per disk), bought them, and have not had any problems since. It would be worth you while to buy Verbatim disks.
Have a great weekend!
Ed Wood