Yes it should work. Though Xlcio really should supply the total watts on the 12v. I suppose they could be saying the the 12v lines are 410 watts (550-140) which wouldn't leave too much head room but should still be okay.
Is just the 5770 new? Did you remember to hook up it's 6 pin pce power line?
yeh the 5770 is chrimbo new, and its all hooked up, see the thing thats makin me think its the psu is that i took my old 8800gt out (which was giving me the same problems as the 5770)and put in the older ati x1650 which doesnt need a 6 pin, and there was no problem, just poor performance. My computer just dies when i play games, like someone switched it off at the wall.