64-bit vs. 128-bit bus
A 128-bit data path allows greater throughput than a 64-bit data path, in the same way that a 4-lane highway allows more traffic than a 2-lane highway. This allows the computer to manipulate large files faster. With a 128-bit bus, you would see better performance from the iMac when you have multiple applications open at the same time. On the fun side, you would also see some enhanced gaming performance.
"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.
If that technical stuff sounds scary, don't worry. There's an easy way to make sure they match. Just buy your DIMMs at the same time as a matching pair, like socks.
Note: This applies only to the original iMac G5 and the iMac G5 (Ambient Light Sensor)—the iMacs in this shape that DON'T have an iSight camera built-in. If you have an iMac G5 (iSight) or an iMac (Early 2006), the configuration of memory is different in your computer and these instructions don't apply.