I wouldn't bother selling it on ebay. It costs money to post it up and by the time someone decides they want it they look at the shipping price and run, and if you put free shipping you either up the cost of the computer or eat the cost of shipping.
If you have a craigslist for your area that would be the place you would get the most money for it. As for the value, I sold a computer in the spring that was maybe an '03 or '04 model system for $50 I think it was. Specs were a little better, the fact that it comes with Office doesn't really up the value much if at all. I'd say at absolute best you'll get $40 for it. I'd just keep it and use it as a server or something. Most people just give them away because they can't sell them.
You;ll end up losing money on Ebay TBH. This PC, I'd give MAX $50. These CPUs haven't been the best to have around, and 256MB is ridiculously low these days. It would have been comparable to 16MB RAM when 256MB was plenty.