Which Power supply should i get for my computer?

The best psu there is the OCZ but any would do for your system. The corsair is quite popular too. I'd go with the OCZ if you can afford it though plus it gives you head room for a possible upgrade one day. The main thing is ensuring the correct connectors, most come with a 20+4 pin connector so they suite any motherboard, the extra 4 are just on the side of the 20 so you can use them or not. The psu rails distribute the power throughout the system and as you can see the motherboard requires the most amperage as it powers all your other components, hence it will take the biggest rail. . . Also you might wanna check out the efficiency Ratings of your psu -whichever you take- those brands you listed are tried and tested so you'll be fine. As for the other two rails i'm pretty sure the smaller is for optical drives and the other is for mostly peripherals and hard drive. Hope this helps mate