I only buy Mac RAM from two online vendors. Too many brick-n-mortar electronics or office superstores simply don't know Macs enough to sell you the proper RAM, especially for older models. I've asked about Mac RAM in some very well-known retail establishments and was looked at like I was talking about a burger.
One trusted vendor is Other World Computing, whom Ralph has already linked. The other is Data Memory Systems (
http://www.datamemorysystems.com/G4iMacMemory.asp). I've gone with either site's least expensive RAM for about a decade and it's all still in service; and we have 15 Macs in the family!
Crucial's RAM is always high-quality, but I hesitate to recommend them to owners of older Macs because of how their RAM finder is programmed. Many older Macs, with modest RAM limits when shipped, can now take advantage on newer, larger models that are still compatible. The faster iMac G4s are among them. The orig spec sheets, not updated since the model was develope, show 1G RAM as the max, but those using PC2100 or PC2700 RAM can today hold and use up to 2GB RAM with complete stability.
Crucial will not show these new developments, preferring to use Apple's badly outdated spec sheets. The other two vendors always show when an increase is possible and test to make sure it works.
I don't think Crucial is bad, just very focused on the larger peecee market. OWC is primarily selling to the Mac users, and people I've talked with at DMS certainly seem to know their Macs. too.
Whatever direction you take, be sure to look up the exact specs for your model iMac ("About this Mac..." --> "more Info...) so you get the proper RAM. The iMAc G4s used three differtent RAM types over their production life.
