Having your OS on one drive and your data on the other helps keep the OS drive less fragmented. If you are on a tight budget, the price difference for having two drives is not justifiable. You will not see major speed improvement for using 2 drives. Keep in mind that you should make 1 partition for the OS and applications, and another one for storage such as musics files, pictures, etc..
On the other hand, a lot of people use 2 drives to backup their data since the risk is higher if everything is on 1 drive.