"Running in pairs isn't required on any consumer level DDR1 or later system. IIRC it hasn't been required since the p1 or 486 era. If your ram isn't installed in pairs there will be a performance hit because the CPU will spread data across both sticks of ram that is in pairs doubling the transfer rate. On some older controllers you might force all your ram access into single channel mode.
The performance hit from doing this is smaller than what will happen if you run out of ram and are swapping though; and isn't something you're likely to notice outside of benchmarks.
The price difference between buying a single large dimm and a pair of smaller ones is typically negligible, so if you're buying new you should generally go with pairs. But if you have a spare dimm sitting around not doing anything else there's no reason not to install it."