The first thing you should note about your motherboard is the CPU socket type.  CPU are made to work with a particular type of socket.  If your board does not use that socket, no amount of force will ever make the CPU and the motherboard work together and your new pc will not work.

The types of memory supported is also important.  DDR2 and DDR3 are the two types found today, with DDR2 the older and slower form of the two types.  The motherboard will list what types it is compatible with.  It will also list the data rates of the memory that it is compatible with.

Graphics are a necessary component and some motherboards will come with on-board graphics cards while others will not.  Additionally, it is important to note how many and what type of graphics cards the motherboard will support.  The on board graphics will work well for simple applications and computing, however games will benefit a great deal by the addition of a graphics card.  It is important to figure out what type of graphics card you will need and then make sure that the motherboard will support it.

Lastly, motherboards support certain types of USB connections and SATA connections.  The new USB form is 3.0 and the new SATA interfaces (for connecting to HDDs) is 6.0Gb/s.  If you are using an HDD that uses 6.0Gb/s, then you will want a motherboard that can use it at that speed, otherwise the motherboard will be limited to SATA 3.0Gb/s.

In summation, it is very important that you first understand what you need when looking at motherboards.  If you are looking for a budget pc, then the older models will work fine and there are still plenty of older components on the market.  If you are looking for something with upgradeable potential, then there are lots of good choices too.

Cases come in a variety of sizes ranging from mini-towers to full towers.  The first thing to make note of however, is the type of motherboards that it is compatible with.  Motherboards are generally either m-ATX (Micro ATX), ATX, or e-ATX (Extended ATX).  However, there are some variations in naming.  Some larger e-ATX motherboards are called XL ATX.  Each case will be compatible with certain types of motherboards.  Larger cases will be compatible with ATX or e-ATX  and smaller may just be compatible with m-ATX or both m-ATX and ATX.

Space inside is also important.  Non-integrated graphics cards are getting larger and cases made for high-end gaming are increasing their size too.  It is always important to check the size of the graphics card and making sure that it will fit in your case.  Also, the amount of 5.25″ external slots and 3.5″internal slots are important.  If you plan on using a lot of storage and want more 3.5″ internal slots for your HDDs then you will probably want a bigger case.  The 5.25″ external slots are used for things like optical drives.  Again, the amount you need will depend more on what you need now and the plans you have for your computer.

Fans are also a big part of cases.  If you plan on using your high-end components to the max, more fans are key.  If you plan on having a basic or budget computers, fans will still be necessary, just not as many.  The cooler the components the smoother they run.  Good airflow and a good fan is important in a case, and the option to upgrade to more fans is something that should be kept in mind.