Serial ata is sata. As you put it, a normal harddrive, you will probally be refering to IDE. IDE has a slower transfer rate, but they do not even use the full bandwidth that IDE has, so Sata will not be that much faster, but you get the thinner cables. The reason that sata is faster is because you can have raid on different channels (with ide it has to be on the same cable for raid, so it is limited by bandwidth, and it can't read and write at the same time.
You will have no problems using one sata harddrive for everything. The only thing is, you may want to create a partition for the games, but it doesn't imprive speed, just makes it a bit cleaner on your os harddrive.