Well... if you self softmodded the 9500... Id consider softmodding to be riskier than overclocking but hey I dont judge.
As far as I can gather, its quite like overclocking a CPU. Crank it up a notch, run something heavy on it, wait until it heats up and see what happens.
My 9800Pro can handle 435/379 for about 1 hour load before i can notice artifacts on stock cooling. I usually use 430/375 for 30-60 minute bursts of gaming with AA/AF... i dont bother overclocking without AA/AF as Im happy without it usually.