Latency isn't important unless you have an AMD chip, in which case it's a LITTLE bit important. You won't have any problems mixing and matching, but it's best to buy ram that matches your current stuff in terms of timings and bandwidth (ie if you pair 3-3-3-7 DDR533 with 2-3-2-6 DDR400, all of your ram will most likely run at 2-3-2-6 DDR400)