Hi Dave, I would replace the DVD as the DVD writer works better if you read from it to the hard drive and write back to it rather than do a DVD copy on the fly like you do with CDs.
You can use either the DVD writer or the CD writer for doing your CDs on the fly, unless your DVD rom is a fairly new one then it probably has a slower CD read than your CD writer as well.
You would probably be better off using Nero 6 as the burning software for CDs using either your current CD writer or the new DVD writer as that will do everything that you need including DVDs.
I know that you have access to Nero 6, (along with DVD ripping, editing and copying programs), so dont bother with the bundled software that comes with the DVD writer and uninstall the current Nero, if its the one that came with your CD writer as that may well be specific for your writer.
Nero 6 is really easy to use for either CDs or DVDs, just remember to select which writer you are using and set it up as your current version of Nero is