Low end cards like the 6570, 6670, 7750 etc usually don't have RPM sensors on their fans.
0 RPM is normal for those cards.
Secondly, the memory is DDR which means "Double data rate"
It transfers data twice per clock. 500 MHz has an effective memory data rate of 1000 MT/s.
When you over clocked it to 1000 MHz, you brought the memory up to 2000 MT/s or double its specs. No surprise that it crashed.
It should be either advertised by its actual clock rate or labeled MT/s instead of MHz. But that is just how marketing is and virtually all memory is sold like that.
In short: 500 MHz is correct.