The best option is to buy the Mac from Apple with the minimum RAM and install your own. Get the iMac with a single 512 Mb module, then buy a 1 Gb module from a reputable seller who guarantees compatibility with the iMac G5 and offers a lifetime warranty. You'll end up with 1.5 Gb for a reasonable price. The RAM installation in the iMac is a doddle.
If you must order from Apple, get the single 1 Gb module, so you have an empty socket remaining allowing you to update it later without pitching one of the 512 modules you paid for...