Most RAM mfrs actually tested, or say they tested the multi-stick packages together, so they should work in a high-performance computer. This is important if you plan to OC your RAM, or plan to try reducing the latencies. If you are going to run a stock setup, buying the matching single sticks may save you some money.
Remember to use memtest86+ when you install ANY new RAM - boot to it from POST without making any changes to the RAM settings in the BIOS. Let it successfully complete its testing before you believe your RAM is good. RAM problems are difficult to find or diagnose - memtest86+ lets you know whether or not your RAM is good at the advertised settings. If you OC, then memtest86+ tells you whether or not your RAM is stable at...