well didn't you say when your 6800GT was artifacting that your room is hot as all hell because your parents wont run any AC? Something like 90F+
Wouldn't be surprised if your card is hitting 90C+ especially since those are more powerful.
Definitely need to look into an NVSilencer or whatever Zalmans latest one is.
But yeah, PZERO also makes a good point...I remember seeing the same thing that that card was supposed to be 460MHz but it comes 470Mhz instead...you need to underclock it to 460MHz and then check with the manufacturer for a BIOS update because that seemed to be the answer for a lot of people