The 600w modxstream claims to support 504w on the +12v rails. Crossfired HD4870s shouldn't use more than 300w so it should be ok I think although you may be pushing it a bit more than is optimal.
If the specs can be believed(and OCZ is a pretty good brand) then it should be alright. I would probably leave the cards at stock though and refrain from overclocking.
You only need 491W to run a Crossfired 4870 set up. You'll be fine. Also OCZ is generally considered to be a 2nd tier PSU. Still good but not on par with Corsair, Seasonice or Antec.