It is possible the G5 does not like DIMMS greater than 1GB each. i.e. one 2GB DIMM may be too much for it to hangle in one chip.
Based on the following it seems to be true for earlier models: http://support.apple.com/kb/HT2248
You got the wrong RAM for a couple of reasons. According to Crucial's RAM finder, the 2.1G 20" G5 iMac should take this RAM:
It's PC2-5300 and non-ECC. PC2-4200 ECC won't work
Return it. I can't begin to tell you the number of posts about getting the wrong Mac RAM on an auction site I've seen over the ~12 years I've spent in these forums.
You do not say where you are. If you give us an idea, we can direct you to respected vendors of Mac RAM that works first time.
When I checked other vendors' site, the PC2-4200 is showing for your G5. Crucial knows faster RAM will clock back so that may be a way for them to limit inventory.
These are the two vendors from whom I've bought all our Mac RAM for over a decade:
Data Memory Systems (New Hampshire):
Other World Computing (Illinois):