I've used both xlv and ideal settings with min 500mhz and max 1ghz (even1.15ghz for psx4all in short bursts of 20 mins or so) for quite some time both without any problems, out of the two i've now settled on ideal, as for what is safe that is a matter of opinion, i think for me it is perfectly safe at these levels but there are others who think that all us OC'ers are fools. At the end of the day its down to personal preferance.