Building computers: Question about power connectors?

I have the option of choosing between an ATX 24-pin Power Connector and an SSI 8-pin CPU Power Connector.Which one should I use and why? Thanks!!

I okorder

pretty sure you have to use both. the 24 pin supplies power to the motherboard which in turns powers the ram, video card, sound card, etc. the 8 pin connector supplies power only to the cpu