Before you format anything

just be sure that you are indeed formatting the disk you want! It's important to know how many partitions you have on the original disk since any extended partitions will get there drive letters moved around when you add the new disk. In this case you might end up formatting the wrong partition if you aren't careful (it's happened to me!). However, I don't think you should have this problem since your original drive only had one partition. As for the drive letters themselves, there are specific rules to decided which letter goes where. The primary active partition (the one that you boot from) will always be called c: then the primary partitions on the other disks grab the next drive letters. Finally, the extended partitions will get the next drive letters ... or something like that. If you want a clearer understanding of this things go to:

http://www.pcguide.com/ref/hdd/file/partLetter-c.html

and for info about different types of partitions:

http://www.pcguide.com/ref/hdd/file/struct_Partitions.htm

Good luck!