i wouldn't think it would work for a 5870 xfire.
1st, the 5870xfire would require a minimum of 600w psu, you are just 20w over the minimum requirement. I might say that your power from the PSU does get distributed all around your pc for the other peripherals to work....the motherboard, the cpu, the hdd, the optical drives, fans, etc. Just having 20W above is really pushing hard on the limits.
2. The Corsair HX620 has 2X PCIE 6+2 power connectors.http://www.corsair.com/products/hx/default.aspxhttp://www.hardwaresecrets.com/article/371
You would need 4X PCIE 6pin power connectors to run a Xfire of 5870. (2 connectors per card)
I would suggest getting a Seasonic M12D 750W psu. http://www.seasonicusa.com/M12D.htm
Unless you might wanna wait and check out 5870X2