You will have plenty of power for games like Crysis 1.2 and World of Warcraft. ATI's official recommendation for a pc system with two 5870's in SLI mode is 40 amps on the 12 volt rail.
Take a look at the power consumption results in this Tom's Hardware review of the ATI Radeon 5000 series cards which includes two different 5870's - one with 1Gb of memory and one with 2GB of memory: http://www.tomshardware.com/reviews/graphics-power-consumption,2708-6.html
A pc with the 2GB model maxes out at 329 watts.
Here's a quote from a post I made just about one year ago:
" I read a technical review of the 5870 over at anandtech.com. They measured the total power draw at the wall outlet (mains). That's the total power used by the entire pc. The test system consisted of an Intel Core i7 920 2.6 Ghz overclocked to 3.33 Ghz and just one 5870 and another test with two 5870's in Crossfire mode. They did the test while playing World of Warcarft. The total power draw for the entire system with one card was 295 watts. The total power draw for two cards in Crossfire mode was 430 watts. I knew the new cards were going to be energy efficient. I'm beginning to think the Corsair VX550 could power a moderately overclocked cpu and two 5870's. It's not that far from the ATI recommendation of 600 watts and 40 amps on the 12 volt rail(s) for Crossfire."