Did you put it back together and closed the case door before turning it on? I seem to remember (but I may be wrong here) that G5s will not boot at all unless the door is properly closed.

If this doesn't do the trick, try heating up the area around the RAM slots with an hairdryer, if they suffer from the bad joints problem this should suffice to bring them back to life at least for a while.

You should also check the RAM you put in is correct (ECC won't work if I remember correctly) , and also try to put in a couple of sticks. G5s are picky about RAM!