I prefer OCCT as a test, as it only takes 30mins to complete the test.
Now, as a dual core is 2 single cores stuck together, one will overclock slightly better than the other, so you want to make sure you're stress testing the weaker core. A good way to find out is run 2 instances of Prime95 (1 per core) and see which fails first. Then, just set the affinity of OCCT to your weaker core.
Keep going until OCCT fails, and you don't want to increase the voltage anymore. If OCCT isn't stable enough, begin underclocking until Prime95/Everest runs for long enough to make you happy.
Also, I would greatly recommend putting your memory on a divider. If not, and a stress program fails, you're not sure if it's down to your CPU, or your memory. Make sure the memory runs at/below 200MHz. Then, when you've finished overclocking your CPU, start bringing your memory back up until it fails the Memtest 86+ test (the one done from a floppy disk - not windows based).
To get the most out of your overclock, you might even want to put your CPU on a lower multiplier, and overclock your memory. Then, mix-and-match the FSB, divider and multiplier to get the greatest benefit from both the CPU and Memory.