the fan noise is caused by the controlled speed. It's louder when the card gets hotter -> faster speed. 250w should be okay for just the x600. Get the latest catalyst from ATI.com and google for ATI Tray Tool which allows you to set the fan speed accordingly. You can get ATI Tool as well (diff version - less bells and whistles). When you boot up, by default, the fan runs at 100%, that's why it's loud, but after the firmware kicks int, it's settled down to lower speed because of the temperature. But when the temp gets above certain degrees, it increases. ATI Tray Tool and ATI Tool (2.5 betta I think, not 2.4) will tel you what temp you're at and what's are the default setting for the fan speed. You can tweak them accordingly. You can also test for artifacts as well.

What you're experiencing may be the cause of artifacts. Check the card for any modding (soldering and flux). Also, the heatsink may not be seating right on the GPU core, therefore, causing it to get extremely hot and resulting in artifacts. Double check to make sure.