A lot of the early ATI cards don't have sensors but it was worth a try.
Since it happened with 2 different cards no sense folowing that path any farther.
Since the MB does not report 3.3v that would explain the earlier reading, there's no sensor for it.
Lets try running memtest+ for at least 6 passes or unless if fails earlier it will take some time.> it's a bootable ISO file burn it to a cd and boot from it or use the Zip drfive version and boot from it.>
http://www.memtest.org/#downiso