mmm... I'm not too familiar with older boards and CPUs, but $500 strikes me as a bit steep. PC components unfortunately don't hold their value well.
I would guess around $300 but don't take my word for it. You might want to include the brand of the GPU, PSU and RAM, and post the specs in one of the other forums someone with a little more knowledge can give a better estimate.
@Popatim, I had that in mind as well but considering it's being sold to a friend, I wouldn't be too concerned about it really. Edit: Scrap that. While I personally wouldn't be too concerned selling it to my friends, that's poor advice. That said, you're spot on.