understanding

Can anyone make this really simple? If I have, for example, a 500W @ 4OHM head, and plug it into a 4ohm cab that can handle it, I get 500W right? What about 8ohm cabinets? Will I get half the power, or can I use two of them? If I found a 4ohm cab I like and a 8ohm, can I run both? What...