The most important thing to remember about memory is to get at least 2 GB for Linux and 4 GB for Windows.
The DDR3-1333 MHz and DDR3-1600 MHz RAM memory are supported by most motherboards, and while the 1333 is enough for HTPCs, 1600 is better for overclockers.
Nowadays, there are faster and faster memory modules available, but you should carefully check what is the maximum speed supported by your motherboard.
For HTPC use, you will not get any noticeable improvement by using faster speed RAM modules.
Kingston HyperX Fury is solid memory for a good price. It has very fast performance and a low heat signature. In my opinion 4 GB is enough for most people, but you can get 8GB if you wish.
You should not use memory modules with heatspreaders, especially with larger CPU coolers as they may not fit under the heatsink.
You should use lower voltage memory modules as less power consumption means less cooling is required. This will also save on the electric bill.