It's probably not power that is causing the issue, likely its either a buggy card, or the motherboard needs a firmware upgrade.
Test over-clocking with each card individually, see if the freezes occur only with one card, or if they work separately just fine.
To make sure power is not the issue:
You can get a simple multimeter for 10-15 bucks from nearly any hardware store/walmart to test the 12v Rails when at full load, see if they drop amperage significantly, that could also be an issue. Setting the cards on different Rails is also a good idea.
Also check with the manufacturer of the motherboard for a bios update, sometimes that fixes issues. Also in the bios you can sometimes set "constant voltage" or "load spectrum" or similar, that should make over-clocking the GFX more stable.
Adjusting the PCI frequency/HT freq, can help on some AMD mobos.