Whats the current rating on the 12V rail? The best i could find for an ultra 600W was 37A which isnt good for a 600W unit, and certainly is not crossfire capable for 4890s, a good 700-750 with atleast 60A is generally recommended for crossfire, it keeps you well inside the efficiency range for the PSU.
So best case scenario its got 38A, likely a few less, thats still pretty weak for a 600W PSU as a good 550W unit is expected to have atleast 40A. For a single 4890 the Ultra is fine, however for crossfire you should definitely upgrade.
Being SLI ready doesnt mean too much, because you can technically SLI 2 9600GTs, it wont take much power so it would be fine, but it isnt what most people expect with SLI.
It depends, a good PSU will just shut off if you try to draw too much power from it like if you were playing a game, a bad PSU will self destruct if you try to draw too much power from it, there is a lot of middle ground between the two but im not sure where that ultra sits in there. It would probably just shut off but you can never be sure.
BTW, here's the dusty PSU I was talking about. It's got four 12v rails, 3 at 18A and 1 at 20A. The repair center that I took my old PC to for a diagnostic test said that it seemed to work fine, but the power dissipated abnormally slow.