Its easy to fit, depending on the case you might have to remove the existing drive cos some Dells come with the space for only one optical drive and you can use any drive it doesn't have to be Dell.
The biggest problems you might have is getting the case apart, some of them are quite tricky to work out how you're meant to open them and if the old drive has a non standard front you might have to remove it and just leave the normal DVD front - if you know what I mean.
Just thought of something else, I did have a Dell apart once and noticed the particular drive that was in this one was quite a bit shorter than I was used to and space was at a premium on this particular one so its worth checking that there is actually room.
If all this is too much for you, you could get an external one. Just a thought.