I have an Apple iMac 24" 2.16GHZ on which I have recently installed Snow Leopard. The machine came with 1GB of memory but Apple recommend 2GB to run Snow Leopard. The machine has a maximum useable memory capacity of 3GB but Apple told me that to maximize the memory I would need to buy 2x2GB cards and not use the 4th GB. Looking on the Crucial Web site it seems to suggest that I could buy a 2GB and a 1GB memory card and that would work. Who is right?

Both are right. Yes, you can use a 2GB and a 1GB memory sticks together and yes, technically, a matched pair (2x2GB) would be better but in this case, not so much. Considering you already have one 1GB stick, it would be cheaper to just buy a 2GB stick. Go with that.