Dimension 4550 DVD-Rom drive

I need a replacement for this drive. But, while I find many similar drives on the net,  I'm never quite sure they are exact replacements. Dell's invoice lists it as: Samsung dvd_rom sd616t (16x). I wouldn't mind upgrading to r/w if I knew how to get there from here. But I'd be happy enough just to have a working rom again. Any help is appreciated.