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?
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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.
The 2009 iMacs should have a memory limit of 4GiB or 8Gib, depending on the exact model. Given current memory prices (which aren't likely to increase until the 2009 are so out of date that memory modules become rare), go for 4GiB.