The new hard drive will be automatically assigned the letter D and the CD/DVD will be moved down to E and F, you can actually have letters all the way to Z.
Seagate has a handy utility that will automatically update your drive access for programs that are looking for a cd in drive D, as the CD is now E. I think it came as part of their diskwizzard software, there may well be other utilities out there that will do the same thing.