Check ebay for something with similar performance and check your local craigslist or something like that. A lot of the pricing will depend on where you live. Also is going to be hard to sell because you are not a company with a reputation and warrantee policy. You may end up having to part it out to sell it.
Personally I'd sell all of the peripherals seperately to make it an easier sell as well as simpler to value (as Pro Llama stated, check ebay for individual pricing).
Also be careful selling used HDD's.
I'd also try selling some of that RAM seperately, I recently sold some 2 year old Balistix on ebay for ~140% of their original value due to the ridiculous prices at the moment.