Can an 2009 iMac have an odd total amount of RAM? I have an iMac 20-inch early 2009 model. There is 2 GB of RAM (2 x 1 GB sticks). 

Can I upgrade the amount of memory to 3 GB, replacing just one of the two RAM sticks?
You can but it's recommended to use matched pairs as you'll get more of a performance boost due to the way the Intel side of things handles dual-channel memory. However, more RAM generally trumps less RAM so it's probably worth it to get that extra Gb of memory.