The capacitors that were in the iMac G5 systems (and many other PCs as well) used a formula that was faulty, and in turn caused the capacitors to be defective. More about that problem here:

http://en.wikipedia.org/wiki/Capacitor_plague

The replacement capacitors you buy should be fine at this point in time. The issue was widely publicized and I do believe that most made sure to eliminate capacitors using that faulty formula.