What divider have you got the ram on just now? I'd stick it on 133 and then try increasing by 10 each time as dreadi says, but i would use Prime95 to test each time (i've not much experience with superPI). I'd keep increasing till prime95 fails within the first couple of minutes, then pull back a few htt and leave prime running for 8-12 hours. If it doesnt fail in this period then you have a fairly stable overclock.