i guess if u can get it stable @ 520(gpu) then u are lucky..
because the new x800pro-s doesnt want to oc at all
atleast with stock cooling/voltage
my cards gpu went only 510 with stock cooling/voltage
with watercooled i got 520 and with more voltage(1,37 @ 1,57) i got
570 3d stable and atitool thought its max was 590mhz..
but it crashed even on 580mhz...
i think that yours atitool artefact search is messed up..
because its not supposed to freeze, but to show artefacts/errors
and any mhz over 520 is very good with stock cooling/voltage
about the mem oc: i think that it will run over 1100mhz 24/7 without any problems
but you should try this out by yourself