graphic card problem

I have 450 w L and C power unit, nvidia geforce 6800 ultra pci-e oc. First i had a 6800 gt oc which died on me a couple of months ago not allowing me to get past my bios screen i sent it in for an RMA at bfg. they sent me back a 6800 ultra oc 512 mb card which is better when i turn on my pc it goes past the post screen but theres a new problem the screen shows lines on it horizontally on post and windows. and during windows it would freeze up and come back normally annually, and would show artifacts on nvidia driver logo preview. it would then say driver failure. i reinstalled the driver but same problem. did my psu kill my graphic cards because i heard l and c is not good? my cpu is amd 64 3000+ at 2.50 oc ghz ram is 1 gb pc3200 512mb (2) 266 mhz oc , 3 hdds ,80gb ,80gb, 40gb
2 DVD Drives. shows same effect without overclock. is their anyway to fix this, cannot RMA anymore because BFG new policy original Receipt required and i dont have it so im stuck with this card until i make enought money to buy a new one