If running newer (as in Sandy Bridge) systems then you will want to keep it below 1.65V, preferably no more than 1.5V. For this reason there are newer kits out there with crazy low voltages. Even 1.65V kit will do fine with SB, only you will have no head room for OC-ing your RAM.
Heat sinks or not? Definitely not needed. Nowadays RAM doesn't need cooling no matter how far you push it and large (tall) heat sinks will only limit your CPU cooler options. Get the lowest one you can.
How much MHz? 1600 MHz is more than enough. And even if you can use 1800+ kits on your rig, cost vs. performance simply isn't worth the OC or your money.
In short:
get a 2x4GB kit with lowest heat spreaders you can find and tightest latencies.I have gone with Kingston HyperX Genesis 2x4GB modules, and have nothing but good words about them. The latencies could be tighter, but the bandwidth in combination with SB chipset is something else. And they fit under NH-D14.
