Hello phoenix,
Don's advice is excellent - as usual!
To present for sale: wipe down with a damp cloth, blow out the dust bunnies, make it look loved, remember what your mother said about first impressions!
A good working mouse and keyboard is a plus, that 500mhz is a distinct plus (assuming your Mac is an AGP model), the monitor is a $25 plus assuming your buyer wants it. See this page
http://the-ish.com/ to get some idea of price; he keeps track of such things. Off the top of my head, you might start thinking $150 and adjust up or down accordingly.
HTH.....Jon