A very important part of your computer building arsenal, is the housing to put it all in. This is the only part I did not have to buy for mine since I found someone with an old case. I took all of the old parts out, and it worked perfect! If you don’t know someone with an old case, there is a huge selection out there with a huge range in prices.
Choosing a case: Most cases will fit what you need, but there are differences in materials, flashy LED lights, and peripheral ports. I do not believe you should dish out a super amount of $$$ for a case, and if you are trying to save cash building a computer, then you most likely want to go with a cheaper one.
Check out the selection of computer cases at Newegg.com.
I bought all of the parts I used (excluding my case since I got mine from a friend) at newegg. I would highly recommend them since everything else I purchased was shipped timely, and in perfect condition.