iMac G5 and RAM: be careful

The iMacG5 has 2 memory slots. But contrary to the PMG5, it is possible to install 1 or 2 DDR modules of different size. But in this case, the memory bus will be working only in 64 bits mode. Only if both DDR modules are identical, that it will work as 128 bits mode. In other words, performance of your iMacG5 will be dramatically impacted if you do not install 2 identical DDR modules.
Warning, identical DDR modules imply both same size (i.e. 256, 512 1024 MB) and same specs (i.e. latency).