Steven is correct. Furthermore, if you run any software other than E1, you must confirm support for V6R1. There are still vendors who do not support V6R1 or require that software to be upgraded first. Also, if you are not on a newer model server, confirm that all hardware will work with V6R1.
If you have never done an i5/OS upgrade, I strongly recommend enlisting the support of your IBM Business Partner or and independent consultant to assist in the process. Upgrading to V6R1 can be a big deal.
-Ethan