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If you have your OS and Programs on one physical drive, and your data on another, both disks can be used at the same time. Your OS/Apps disk for loading things from the hard disk into RAM, whilst transferring data to and from your data disk at the same time.

If you're going to do this, remember to put the second disk on a seperate channel on the motherboard.

If you motherboard supports RAID, you may consider using it. The levels of RAID usually supported by Desktop motherboards are:

RAID 0 - Striping, data is written to and read from both disks simultaneously, which makes them perform very, very fast. The down side is if one of your drives fails, you lose all of your data. Requires 2 or more disks, and they usually have to be the same size but not always.

RAID 1 - Data is "mirrored" onto a second disk. This means that if one physical drive fails, you still have all of your data on the other one. The read speed of the data doubles because the computer can read from both disks at the same time, but writes are the same speed as one disk. RAID1 would double the price of your existing capacity, but if one disk dies and you don't have a backup you won't lost anything.

RAID5 - The best of both worlds. RAID5 requires 3 or more disks that usually have to be the same size. One of the disks if used for parity if you experience a failure - so you get the total capacity of 2 disks out of 3 (or the total number of disks, minus 1 for parity). With a good RAID controller, a RAID 5 array can be very fast AND redundant.

RAID 6, 10 etc and all the other RAID levels don't really have many applications on desktops or workstations, and are the domain of dedicated servers only.