i had almost the same problems as you (graphical glitches, aero dots) on my old 260 gtx.

the problem was NOT due to gpu, it was due to RAM

even though the ram passed memtest 86+ for hours and several cycles, i ran it through (then) vista memory diagnostic, and it failed.

try running your ram though windows mem diagnostic rather than memtest and see if it passes

FYI, i had this problem initially with a 4870 512mb, then RMAed for another one, then i got a 260, RMAed again and finally figured out i can't trust memtest.

BFG is a good company and they are correct that the chances of you getting 5 bad cards are a billion to one.

probability states that the DOA cards were probably due to you failing to take the right procedure in installing the cards (aka maybe you didn't use ESD protection, etc...) and that caused doa cards.