I tend to agree - if it's not a "specific intention" PC (ie a gaming machine, or a graphics intensive work machine, etc) but just a net + recreation PC (like mine is) then it might pay to buy one of the shelf...
I bought a used Dell Optiplex (tower) for £200 which came with a 2.8ghz P4, 1.5gb RAM, 60gb HDD (wasn't worried as I had replacement drives to go in it), DVD-ROM, DVD-RW, mouse and keyboard (no monitor) and was set up with XP SP2 and ready to rock out the box - on-board gfx and sound.
All I did was stick in my Hard drives, reinstall the OS, add an optical soundcard, and plug it into my monitor - sorted!
It's been faultless since I have had it (2yrs now?), only had to reinstall a few times, usually to my own fuck up

Can't stop messing around in the registry

and for a virus that fucked me so hard in the ass, my teeth were brown....
But if you choose to build, don't go less than 2.6/2.8/3.0ghz P4, 1gb RAM minimum, and at least 120gb HDD - trust me, you can fill these fuckers up proper quick!!!!!!!!!