quick question - is this machine getting its power through a UPS ? If so , has it ever been required ? ( ie power voltage dipped or off and the machine kept running ok ) so that you know it works fast enough ?
If you don't have, or it doesn't, then get a decent UPS. Otherwise what is on your disks may be scrambled on a power droop or loss.
When you add a disk to a machine already running windows, usually you can just mount the drive in a slot, connect the power and data cables, and power the system up. You may need a y power splitter cable and you will need the controller data cables to plug into the motherboard and drive. Make sure the external power cord is unplugged when you are installing the drive and connecting cables ! Also, touch the bare metal portion of the case to discharge any static from your body. Most disks are already formatted, so the OS should see the drive right away and you can use it.
If you are going to move the OS and programs to a single disk, you may notice a slight slow down in responsiveness over the raid 0 array.
i have my data files on a one disk ( some are on external eSATA drive), OS and programs on one disk, and scratch on the 3rd disk. Separating this way makes it easy to keep my backups organized as well as give some speed improvement. Some suggest placing programs on their own disk, but i haven't felt the need. What can help some is making sure they are on different controllers, particularly the OS/program disk and the scratch disk if possible. In your case, i would think about moving the data files off the raid 0 to a separate "data" disk, keep the OS and programs on the raid array where you want speed, and have a separate small scratch disk on a different controller if one is available on the motherboard. You could try a partitioned disk with data in one partition and scratch in the other and see if that is fast enough before you add yet another disk.
Getting the separate scratch disk will likely help the most on speed with PS.