Everything holds value differently. I would suggest trying to look up similar spec systems on ebay and what they sold for. Then decide is it more important to get rid of at your asking price or do you want to get what you can and just get rid of it.
Off the top of my head, I'd say $4,000-$5000 isn't unreasonable for that... but it can be difficult to get as your potential bidders aren't going to be too many (I don't think, because they'd potentially have to need Server Enterprise, Exchange, and a tape drive. You might sell it without the licenses... or list it with a price for the hardware only and let the software be a bit of a bonus. (bidders who understand what they are getting would likely be willing to up the price by $2000 or more as I believe Enterprise sells for $3000). And you probably can't sell enterprise separately as I suspect it's an OEM version.
