If your psu supplies an 8pin, definitely use the 8 pin. But no need to use another 4pin. I've never seen a Mobo like that, but if they truly are both cpu power headers, 8 pin would provide plenty of power. Even just one 4 pin connecter would suffice.
No problem. That is confusing, normally boards have one 8 pin connecter which will accept either one 4 pin or one 8 pin depending on what your psu supplies. Yeah my board has a similar connector for SLI setups, which surprisingly is delivered through a SATA power connector. You are absolutely correct about the extra power connections for the gpu. However, what your missing is the fact the motherboard also delivers 75 watts of power through the PCIe slot. This is why some low end cards dont need any external power connectors at all. The power is taken from the 24pin motherboard connector and delivered to the card through the PCIe slot. The 6 pin power connecter on your Mobo is just there to prevent to much voltage drop across that 24pin connector if its trying to supply 75 watts to several cards. Make sense? And keep in mind SLI doesn't work on OSX, but I assume you'll be using your SLI setup for mostly windows gaming.
Yeah, well that sounds like a good route you took. The Radeon 7xxx series compatibility is sketchy at best right now. Plus you already bought the parts, so I think its to late to change your mind, but you did well.
