Few Questions about Asus Maximus V Formula

Hello ,

I'm planing to buy a Asus Maximus V Formula Motherborard and i have a few questions .

1 . I saw that it has an 8 pin plug and a 4 pin do i need to have both of those cables i have a older version of Corsair 800GS 800W PSU which has only 1 4+4 pin (8pin cable) ?

2. I saw someone on the forums was talking about it bing unstable i you dont have the extra 4 pin pluged im planing to overclock my i5 3570k about 4.6 GHz (something like that) ?

More Specs :
16 GB Corsair Vengance RAM (4GBx4) 1.5 V 1600MHz

Asus GTX 560ti DirectCU II

Corsair 800D Case

i5 3570K

Corsair 800GS 800W PSU