Yep it's almost guaranteed that the PSU is cactuar, and this will be the primary issue that prevents it powering on... However it is likely that it has logic board issues or will have them eventually. Unfortunately, the PCI-x G5's were complete and utter garbage. I have a 2.0DP that makes a good shelf for this reason... It's unstable and unreliable as hell, and sometimes dumps a CPU when it feels like it, rendering it only a single-processor machine. In the end I opted not to fix it and simply replaced it with a PCI-e 2.0 dual-core instead given how far the prices of 2nd had G5's have dropped. I can't say a bad word about this machine... it's fantastic, and appreciably quicker than the one it replaced.
Not sure about the U3 controller... does it use normal feet or is it a BGA arrangement? If it is BGA then I beleive it has to be essentially flowed onto the board with localised heat. If its normal pins, well one may be in luck, however I beleive part of the issue was simply that the memory controller itself was complete and utter rubbish... just like the IDE controllers on early B+W G3's.
