Well first of all have you ever done so and I don't mean calling dell either I mean going to the store and buying each nessesary part and putting it all together yourself.
I have and that computer just so happens to be the one I am using right now :)
I thought about doing it all custom, but to expansive, so i got an older computer and have been building it up top to bottom.
Building your own computer is always cheaper, the sellers of pre-built computers charge extra to gain a profit from buying the parts and putting them together. It's not hard to put together a computer, most things are self-explanatory. I recommend Newegg for parts, they're very cheap and reliable. If you decide on building your own computer, post the parts you are going to get to make sure they all fit together, and your system doesn't bottleneck on a single component. (i.e. a 3.5ghz hexacore on a 200w PSU will severely lower the computing power.) If you give me a set budget, I'd be happy select parts for you :).