The case is probably the most attractive part of that system => although a "... valid copy of xp ..." will also add to the value IF you have the XP installation CD.
The case can be purchased new for $50 ... so it won't add a lot, but it does make the system more attractive than a generic "white box" [
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119047 ]
Looking at recent completed listings on e-bay for a name brand (Dell) P-IV 3.0GHz systems, they're closing out at prices varying from $50 (Optiplex GX280 SFF) to the high $100 range ($150 - $180), with a lot in the $135 - $175 range. This is without a monitor; and in most cases without an OS (although a COA is usually on the box, so anyone with the proper installation media can install XP).
A generic homemade PC isn't likely to sell for as much as a commercial system; but you just never know on ebay & Craigslist. I think $100 - $150 is a reasonable expectation for the system ... more on the $150 end if you have the XP installation media. These prices are for the system itself ... no monitor/keyboard/mouse included [Including those will add $30-$50 to the expected price].
Certainly wouldn't hurt to take a couple pictures and write a nice ebay ad & give it a shot. If you don't like the requirement to box it up and ship it, then just list it on Craigslist for your city (although I think ebay is more likely to result in a reasonable price).
