memory voltages for overclocking.?

Hi just wondering if anyone could tell me what the limits are for my certain memory when it comes to overclocking.

my ram just now is Corsair XMS2 DHX with the domminator fan 5-5-5-18 stock 1.8v ddr2 800Mhz

im currently ar 1.88v @ 900Mhz but aim to get to 1ghz

thanks

Upto 2.2v without active cooling for extended time I would consider safe.