Incorrect. Apple says the RAM must match each other in chip organization and latency settings as well as speed
http://docs.info.apple.com/article.html?artnum=300082
""Composition" of your RAM
Composition refers to size, speed and devices on the chips you're using. If you want a 128-bit data path, you need to match composition of the two DIMMs you use. For example, if you have a 256 MB DIMM with latency of PC3200 - 30330 and 8 devices, you'll want to install an additional 256 MB DIMM with the same characteristics."
Saying "It's PC3200, it'll work" is like saying "This carbouretor should fit your car, it says Chevrolet on it and your car is a Chevy"
Having said all that, two more things. The speed advantage of matched modules has been reported to be insignificant in real world performance. You'd be better to put in a 1 Gb module for the extra RAM.
Secondly, the iMac G5 is THE most picky Mac made, when it comes to RAM compatibility, I would recommend only buying from a reputable seller who texts and guarantees compatibilitywith the iMac G5. I would not advise a new Mac owner to install PC generic from newegg, who do not state compatibility on anything.