It does not matter if you have a large Video collection and all the movies and tv shows ever released, if you dont have the right hardware your system will suffer from video frame dropping & buffering issues or horrible sound problems. Below we will give guidence on whats needed.
This is more of a personal choise as its an individuals preference as to what look the case has is ok. That said there are some things to consider though.
Make sure there is plenty of ventilation and and at least two areas for fans.
When choosing a case for your HTPC system make sure you consider where it will live, ie will you keep it with your satellite reciever, dvd / blu-ray player and TV when you have decided where you will keep it you can decide a style that will better match the rest of the equipment.
Heres what to consider when choosing hard drive's for your HTPC system:
For the operating system as you would want it to boot up as quick as possible you can use SSD (Solid State Drive), these drives have no moving parts and are therefore quicker than normal Hard Drives. SSD drives works with windows 7 / 8 and Linux, Using windows Vista or XP is not possible without bios & operating system alterations. The biggest advantege in using SSD drives is a big speed / transfer rate increase. Although you can store your video collection on the drive that the OS is installed on your better off using a seperate drive to store your video collection.
You can also install The OS on a normal HDD, if so use 7500 rpm over 5500 rmp for a speed increase.
Once you have your OS installed you can set up another internal drive to hold your Video collection, this can be either a standard HDD or a SSD one.
You could also go down the route of attaching a USB drive to hold your video collection, again this can be either a standard HDD or a SSD one.
In all cases for standard HDD make sure you get low noise and low heat dissipation.
When choosing a hard drive, how much money do you want to spend and how much disk space do you think you'll will need. for your video collection choosing between 1-4 TB drives is a good choice and will let you store into the future as well.
Onboard graphics have improved a lot over recent years, and while they are not upto playing new games on, they are more than adequate for anything an HTPC can throw at it. If you have a Blu-Ray drive then you can offload some of the load to a dedicated card that supports it.
You wont want to put a huge GTX 295 in your case blocking the airflow. If you're on a budget and not sure what you will need, you could just purchase an appropriate motherboard with onboard graphics along with power supply. This way you can upgrade in the future if you want.
If you have an older card put away somewhere it should work just fine, if not then there's a lot to choose from out there eg: ATI or Nvidia for example.
Onboard Sound is getting a lot better these days, and most people will not notice any difference. But if you want to buy a sound card then there is a card for pretty much any price range, and you'll find that everyone has a different opinion. Sound cards come in all shapes and sizes, and can range from PCI, PCIe, USB, and IEEE 1394.
As this is a dedicated HTPC system you will want to be using a multichannel receiver with 5.1 or 7.1 speaker setup or using the sound setup from your big screen tv via the HDMI route.
When choosing a motherboard make sure it has onboard graphics and sound, as these are quite capable of delivering whats needed for HTPC needs. Make sure that the motherboard is capable of 4GB+ memory to allow later upgrades, also when choosing the CPU make sure it hase a minimum of two cores, but if you can afford more cores then thats better.
AMD CPU's are generally cheaper than Intel but are usually set to there max capabilities and cannot be overclocked by much.
Intel CPU's although more expensive than AMD, are not set at there max capabilities and can usually be overclocked.