Hi, I have the same problem, horizontal artifacts appear all over the screen whenever a window is opened or anything of the sort. Although it doesn't stop me using my Hackintosh it is incredibly annoying. It doesn't seem to be a common problem either as this seems to be the only thread I can find via google relating to it atall in any depth.

If you make any progress atall I would love to know.

System -
Intel core Duo Quad q6600
2x Geforce 8800gt
4gb (2x2gb) DDR Ram
Asus P5NT-Deluxe Motherboard
Netgear Wifi (forgotten the model No, Atheros chipset)