Hi ray, that can be used for both single and SLI cards, but is 1000w overkill?
Use the extreme power supply calculator how much your currently using, and either add 50-100w or include what hardware you may add in the future. Otherwise when you turn your PC on, the lights in your street will start flickering everytime you turn your PC on!
The difference between ATX and ATX12v (otherwise known at ATX v2), is that ATX v2, supports the newer motherboard formats that have a 24pin instead of a 20pin molex connector for the motherboard. You'll generally find the 24pin sockets on the newer motherboards with PCI-E, which requires a lot more power than AGP. By default, the ATX v2 will also come with SATA power connectors, and they are more energy efficient than ATX. I would go for ATX v2/12v since they are backward compatible and will work with the older motherboards.