If it cant pass the tests, then it would indicate that one or both of the sticks are faulty.
Remove one of them and re try the test, if the same happens then try the other stick on its own and see if it passes.
One should pass, as its VERY uncommon for both sticks to go at the same time.
PC8500 is faster RAM and it can be matched to the frequency of the CPU which means it runs better for Overclocking. If you dont plan on overclocking then its not the end of the world, but as one of your sticks are bad then its not worth putting PC6400 in, just get PC8500 as its about £30-40 for 2GB now
So run the test on each stick independantly and see if one passes
(if none pass then, try another set of DIMM slots as its not uncommon for these to go too)