Should I buy a matched 2x16GB kit instead of adding two more 8GB sticks?

Yes, the matched kit is better. Adding two more 8GB sticks to your existing pair often causes headaches because production batches vary, even with the same brand/speed/timings. The memory controller works harder with four sticks, and you might get stuck at slower speeds or random crashes. A 2x16GB kit guarantees two matched sticks, leaves two slots open for future upgrades, and runs better in dual channel on modern boards. Sell the old 2x8GB sticks for peace of mind.