550 watt Odin is rated at 41 amps at 12 volts. I conservatively estimate 10 amps for an OC 'd CPU, around 22 amps for a pair of 5850's, 4 amps for two HD's and an optical, and an amp for fans. That's a high side estimate of 37 amps.
It should work but you won't have much in reserve.
Try to go for atleast 600W
Since new card are remarkably lower power consumptives so 600W will do the trick
I don't think it's a good idea to put second 5850 on 500W
But you can safely put 9800 GT on it