You can take a look at the approach I used for an HTMC build here http://www.overclock.net/t/1319808/silent-htpc-build
Another good source which I think has all the information you could possibly want to ask in building an HTPC is here http://mymediaexperience.com/getting-started-with-my-media-experience/
The guy who created this website basically does everything for you but build the system for you. The website has live templates for building an HTPC depending on your budget and your needs.
The reason why some people are asking for a budget is because they are making the assumption you will be building a new computer system or recycling components from an old computer to create your HTPC? Or are you planning on using your current desktop and have it work as an HTPC as well?
Since you have asked the first question of what is needed to create an HTPC, we in-turn need to ask a few followup questions to better serve your interests.
From your initial posts its sounds like you want to build two systems... A storage/library area which can sometimes be referred to as a Network Attached Storage (NAS) to hold all your movies, music, games, etc. And the second system would be an HTPC to stream and view/record content to/from your NAS. Some people who are on a budget can use their HTPC as a NAS too but I would try to keep them separate if at all possible.
If you are planning on recording TV shows from a cable company you will need a TV tuner.
If you are planning on backing up your private movie library from DVD, BD then you do not need a tuner. You just need the free software out on the web to backup the movies to your harddrive. I am guessing this is what you are referring to as free software in your last post?
Anyway to build the HTPC the cost of component can range from about $150 to $400. It all depends on what components you want and what are the uses for your HTPC. That said, $300 should be more than enough to cover all your needs for any HTPC build to stream movies, and play some small ROM games. If you go for the latest brand name components it will cost a bit more.Sample build
This is just a sample build, you can switch out components as needed to lower price point etc. Like if you want a separate SSD for your OS. Some of the items may be overkill for what you need but at least this build should handle most of you are looking for. I would also shop around for sale on items to save you a few bucks.
I hope the info I provided give you the steps to move forward. I am sure others will chime in as well for additional hints.