You can probably get more for your money if you build it yourself. But it is not like getting a $500 P4 is a bad place to start then you can make up grade.
Unless you want to build it for the fun of it, is there still a signifigant cost reason to build it.
I know when I bought my last computer and any computer that could do long division was over $1,200, and you could make a rocket ship if you made it yourself for around $600
is it still worth it?