I think it will be very, very close. It may come to how much the PSU can produce over its rated 12 volt capacity.
Figure about 10 amps for the CPU, 20 - 22 amps for the 2 video cards, and at least 2 - 3 amps for drives and fans and right at a peak 3D gaming load, you are probably operating the PSU very close to the 100% load on the 12 volt rails.
I agree, it will be very close, perhaps to close for comfort.
Running it through the eXtreme Power Supply Calculator Lite, using a few assumed values for your system and 25% capacitor aging, puts your recommended minimum wattage around 700w.
In your favor is that Seasonic makes excellent quality PSU's.
They should have no issues running right up to and a bit beyond their rated specs.
Also a plus is that ATI only Recommends a 600w PSU for CrossFire 5870's.
Against you is that your current PSU, while of excellent quality, is of an older design.
Many current quality 500w PSU's will have no issues matching your +12v output.
Your current PSU also has only half the required PCIe 6 pin connectors and, due to it's age, is not operating as well as when it was new.
Add the calculated 700w requirement and things are not looking so good.
I would recommend not trying it.
Get a new Corsair, Seasonic, PCP&C, OCZ or Antec PSU before adding a second 5870.
According to ATI a modern, high quality 600 watt power supply is sufficient to power a system with two ATI Radeon HD 5870 video cards operating in Crossfire mode.
That recommendation is valid providing there is sufficient current (amps) on the +12 volt rail that delivers power to the cpu, video cards, and other components. A modern high quality 600 to 650 watt power supply will have a +12 volt rail with current rated at 50 amps. For example, my brand new Seasonic X-650 Gold Certified power supply has a +12 volt rail rated at 54 amps.
The problem with your power supply is the +12 volt rail rated at 36 amps. That's actually less current than a modern high quality 500 to 550 watt power supply. As other have already pointed out it is just too close for comfort. Since you overclock and already have a 5870 I am guessing you are probably a gamer. If that is the case, then I would bet you would probably experience problems during gaming if you tried to add a second 5870.