I also have a Powerstream 520W, running fine on mine. Though I've had to dial down my OC on Vista a bit, since I can't seem to set the fan settings through Rivatuner like I can with ATITool. Stable at 750/999, but not at 804/999 (Rivatuner can't seem to hit 806).

Are there any OCing tools that work in Vista other then Rivatuner?

BTW, the 2 4-pin connections that make an 8-pin on the Powerstreams are the P4 and +P4 power connectors. They are meant for the motherboard, NOT for the video card. Do not attempt to plug them into the video card, since they shouldn't fit anyways and you'll end up physically damaging the plug by forcing them in, assuming the different current/voltage doesn't damage the card also.