The only way to work out if its worth it or not is look at how much you can sell the parts fro seperately. You would easily make more the $100 back on all of that and you probably would even in 6 months to a years time.
i would say its a bargain, more of an internet build though, to upgrade you would need to change the motherboard first. i think you'll be disappointed with most modern games, they will play slow and default to low graphics. I personally would rip his arm off, take it apart and list it all on ebay.