Feature Test on Vantage draws the most current and runs the GPUs the hardest.
It doesn't sound to difficult to run two tests to narrow down the source of the issue.
First run the test with both cards at stock speeds. If the test is completed successfully it was your clocks. If the computer still shuts down, it probably is the current draw.
Second try running the test at your over clock speeds but only one card by dissabling Crossfire or removing the second card.
I ran 2x HD4870s my self. I found it a hot, noisy affiar, and ended up not liking it at all and just running one of the cards. System kept setting off the over-draw alarms on my back up power supply.