heh, for a minute there I thought you wanted to run two 590's in SLI on a 600W PSU, then I realized you were listing out the motherboard specs.
Anyways, the 550Ti has a TDP of 120W, so two would be 240W. The processor has 125W TDP. Not including anything else that would draw 12V you need at minimum 365W on the 12V rail.
If I found the correct PSU online, your PSU can put out 38A on two rails. That is 38*12 = 456W. I would say it should be safe so long as you don't OC the processor.
Based on the review I read, the Ultra X-finity is a quality PSU so you shouldn't have to worry about it actually putting out what it claims it can.
That's pretty much What I thought. It's a dual rail PSU I thought it would handle 2 550 Ti's okay. Thanks.