See my post above
If you are smart, well-connected, and your time is worth nothing, I'd understand why you'd want to build your own.
But for just about everybody else, the sum doesn't shake out.
I cost pain and suffering at time-and-a-half or better.
By the time you've added up the build, the frustration, the crashes, the fixes, the troubleshooting, the coffee...
you'd be better off buying something custom made with a warranty, that works, out of the box.
I find a lot of the time, some of the bits I'd like aren't even available as a loose item.
And if you think upgrading little by little is an option, think PCI...AGP...PCIExpress, and 64-pin...128-pin...No Pin Anybody's Ever Heard Of, and DDR...DDR2..DD3, 400MHz FSB...800MHz...1600MHz
You soon realise that you are in a loop of carefully planned obsolescence.
Buy the box, upgrade once, dump the box.
This is the world Wintel have crafted for us.