yo man, i have a 550watt PSU and running a 1950pro ocd, e6600 ocd and a couple of cdrom drives and neons here and there and 2 hard drives; dont forget the fans.
Honestly, i dont think that its your PSU. an X1950pro with 512mb mem has a min power requirement of 450watts, uve got 500. get the latest drivers; CCC7.6 and try adjusting the refresh rate. in CCC you can force refresh rates, dont that would case your screen to blink.
if you have a second psu, hot wire it and run only the card off it. if the blinking stops post back saying i'm wrong. if it doesnt stop the problem and you tryed what i told you then its the screen...highly unlikely