I'm with Jbaz I believe (that's the motif for a while it seems). As a whole, this is a build that most users want today as a budget build. If you build it right again, you can get a bit of profit. Most people just want to cruise the internets, so give them the hardware to do it and set them up to not fail.
$200 + $100 for W7 HP OEM, and I think you have something here. Hard drives are pretty much preciouses now, so if you have one...fork it over to seal the deal. I and another geek friend at work had a guy balk just today at a $600 for a build, so go softly. If you can get a working computer out of this for $400, I think you're in the clear. (case, PSU, OS, support...people don't like not having those)