Hmm...i think I might have a clue.
Because your processor was overclocked before you added an extra 512mb to make 1gb and it was stable, does not mean you can insert another 512mb stick (still overclocked) and expect it to work, since the RAM will be talking to the CPU differently now because theres more of it, and the timings might even be different.
So you'll have to unclock the CPU with 1 stick in, and then start the overclocking process again to get it stable with 2 sticks in.
Might be the same thing with you ExGamer
I hope this helps