Several studies were done when the dual 4870X2 cards first came out and they all came up with the same facts.
A 4870 X2 videocard and test system draws approximately 177W.
Energy consumption jumps a lot when the video cards are stressed under a 3D gaming load in 3DMark06. In this case, the total system power draw for the Palit HD 4870 X2 test platform rises to a peak of about 421W.
That's a change of about 245W when this dual card is working at its peak. Keep in mind that some of this additional power is being drawn by the CPU and memory, but it looks like each of the two GPUs on the card is approaching 100W of power burn at least.
As a general guideline, since power consumption will vary with the demands and components of individual systems, but it's fairly obvious from these numbers that as a minimum a 600W would be recommended, however, once you add in the variables like additional drives and other power draining peripherals I would recommend an 850W with a single rail with a high Amp output on that rail.
OK, if you have the kind of money Alan has then go for a 1200W or even a 1500W SilverStone Strider.
I have been reviewing PSUs for Enermax, SilverStone, NEXUS, NZXT, Sentey, ULTRA, Seasonic and others and I always test every 850W or higher unit with HD5870s in CF or GTX570 or 580s in SLI on an i7 930/950 test system with an OC of 4.0GHz and have yet to find a single 12v rail with at least 58amps and 850W unit from any of these respected companies that would even break a sweat under my test loads.
So, save a little cash and take it a little easier on your electric bill at the same time and before you know it you will have enough saved up to upgrade that card.