yes building a whole computer is usually better than buying a pre-built machine if you are willing to put some effort into it. manufacturers like DELL, HP, Compaq, Emachines, Sony use low-quality parts espescially the motherboards.
with custom you can pick exactly the things you want and not only do you get satifaction from that , but you can build a new custom compter very inexpensively these days, plus NO REBATES to deal with!