What kind of heatsink are you looking for. Most commonly you'd be looking for a heatsink for the CPU. Most CPUs come with a heatsink that is decent but not necessarily great. You could also be replacing heatsinks on a GPU, or even on some other components. I'll assume a CPU. The first thing you need to do is narrow your selection down by socket type. Which socket type your CPU plugs into on the MOBO will limit what type of heatsink can be mounted on the board. Once you determine this, decide how much airflow you are going for. Some heatsinks have no fans, some have one, and some even have two now. For a CPU I'd use at least one fan. Also decide what material you want to use. Most heatsinks will be copper or aluminum. Copper is the better heat conductor and should be used when possible. There are also a selection of solid heatsinks and heatpiped heatsinks now. The heatpipes dissipate heat better but are harder to install correctly. Also keep in mind the size of the heatsink. Some of these things are massive and you have to make sure they will fit where you are putting them while maintaining proper airflow. I sometime worry about the size and weight of these things hanging in there with no support but what a thin MOBO base provides. Sometimes bigger isn't always better. You can use a smaller lighter heatsink with the same results if you make certain the rest of the case has decent airflow and plenty of case fans. Also keep in mind that you want to use a good thermal compound between the CPU and Heatsink. I don't like to use the thermal pads that come attached to many heatsinks. Arctic Silver has been a highly recommended product in the past. It's still one of the best out there, but there newer technologies like OCZ Freeze that will out perform it.