For the most part, RAM is RAM. You can buy PC3200 RAM anywhere, but as mentioned above you must buy in matched pairs (same speed, same size, and preferably same manufacturer) for the G5. A lot of newer PCs are going that way as well -- they don't NEED RAM in matched pairs but they're optimized for it.
As with any RAM purchase, it's wise to go with a name brand (Corsair, Kingston, Crucial, PNY) and avoid no-name, store brands or anything that was "manufactured in one of the following countries". Look for the best price you can get on name-brand PC3200 RAM and you should be fine.