1. #1 is the operating system.
This is often added as an after-thought. It is low on the list of computer parts, as there is really not much of a choice to be made here. It boils down to...Windows or linux? The only way you will screw up this choice though is when it comes to choosing between 32-bit and 64-bit windows. 64-bit is the way to go unless you own a lot of 32-bit software that you don't want to have to replace.

2. #3 is the DVD burner.
This is way near the bottom of the list, as it has like zero effect on the performance of the computer system. It's only used to install software and burn stuff to media.

3. #6 is the hard drive.
The hard drive will affect performance somewhat...but not nearly as much as other components like the CPU, video card and RAM in particular. And unless you go for a "green" model of hard drive, then (just like the motherboard) it is hard to choose the "wrong" hard drive. Just about all newish hard drives are 7200RPM with a good amount of cache and storage space.

4. #4 in any system is RAM.
If you don't have enough fast RAM in your system, the performance of the system will be degraded significantly.

5. case
While the case won't really directly affect the performance of the system, choosing the right case is still important. A properly designed case doesn't just hold all the parts. A properly designed case will allow for efficient cooling of all installed (active) components. This means choosing the right case will help you build a cool, STABLE (cooler temps help with stability) and quiet computer system (efficient cooling is also quieter cooling...and most of the noise of a computer system is generated by cooling fans

6. Motherboard
a printed circuit board containing the principal components of a computer or other device, with connectors for other circuit boards to be slotted into.

7. Sound card
A sound card (also known as an audio card) is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs.