Thanks to a lot of good advice from here, I have completed my first build (except OS install). The one thing I had to guess at was plugging in cables from the PSU.
To that end, I seem to have an extra, an 8-pin one from the group of cables that are bundles together in my modular PSU. Is it necessary? I plugged a red "PCI Express" cable into one of the two modular red ports of the PSU and the other to the GPU (4850). Everything powers up and posts, but not sure if something is underpowered that I won't realize until later.
Your Motherboard has a 24pin power connector and a 4 pin power connector. Some motherboards have a 24 pin and an 8 pin. That extra 8 pin cable is likely to be there to accomodate those motherboards. The only other possibility is that its an 8 pin PCIE power cable for graphics cards that use 8 pin pcie plugs.
Yeah, I looked at the specs of the PSU, and there is a 6+2 pin PCI-E connector, but that would have a breakaway. It also said there was a 4 pin AND 8 pin 12v connector, which means that 8 pin, unless your motherboards CPU power connector has 8 pins, is not needed in your setup.