Should I buy a 2x16GB kit or mix a second stick with my existing one?

Yes, get the matched pair. Mixing a second stick with your existing one might work, but it’s a gamble—different batches or timings can cause instability or slower speeds. A kit is guaranteed to work together, and with only two slots, matched sticks give you dual-channel performance for a noticeable speed bump. The extra $20-30 is worth not troubleshooting random crashes later.