Yeah you need both plugged in for the card to work. If it only has a 6-pin it probably isn't powerful enough for your card. Don't try it until you (or we) confirm your PSU can handle your card. Individual cards vary a little but AMD recommends a minimum 500w PSU. What is your PSU?
If the power supply can deliver its rated ~450-480 watts at 12 volts, the card would run with Adapters. You will certainly want both connectors (6 and 8 pin.). The power supply has the 6 pin covered, but you need an adapter to get the 8 pin.
In the future, budget on anything but the power supply. A bad power supply can take out the full system if it fails.
was asking why it needs both cables as i was hoping someone would be able to answer me and then maybe i would learn something about gpu's i obviously dont know.....
That card needs around 175W of power, which is more than a single 6 pin can supply, and the 8/6 pin combo probably worked best. It's a bit older card, where 8 pin was still popular. I think all cards now use only variations of the 6 pin connector, to standardize it a bit, but in the early days, cards had 6 pin, 8 pin, some just put molex plugs on the card, they were all over the map.
5850s usually come with two 6-pin power connectors. 6-pin connectors are capable of supplying 75W, 8-pin connectors can supply 150W, and the pci express slot itself can supply 75W. You normally won't see an 8-pin connector on cards that use less than 225W, and 5850s use considerably less than that. It's possible ASUS opted for an 8-pin instead of a second 6-pin so heavy overclockers could get enough juice. That's just me speculating, but you will need to use both power connectors.
You will be glad to know this psu blew and I am now needing a new one any suggestions for running this gfx card, its on a m5a99x EVO r2.0, with a amd fx 6100, a thermaltake water performer 2.0 and 3 Molex fans.