Checking out OEM computers you often see that you can order the same amount of RAM in different configurations. For example, if you want 4GB of DDR3 you can get that as 4 X 1GB sticks or 2 X 2GB sticks.
There's a practical reason for that: If your motherboard has 4 slots and only two are filled, you can add memory without discarding your existing memory.
Beyond that practical consideration, is there anything else to be gained in choosing, for example, 2 X 2GB vs. 1 X 4GB? Better performance, for instance?
Well if you're only using one memory stick you're only going to have it in single channel mode which is less bandwidth then a 2x2gb setup. Typically the larger 4GB sticks cost more individually then a 2x2GB setup, also the w/ more memory modules you're using more power. As you said there is always the upgrade option to keep in mind.