It would help if you told us how much you paid for it.
I assume you overclocked this rig quite a bit. In that case, if the components like CPU/motherboard are fairly old (1-1.5 years), they would have deteriorated much faster than if you had used them at stock. That would fetch a lower price.
My estimate would be around $600-650.
btw, why sell anything beyond the cpu/mobo/RAM/display? Everything else could be reused (like the HDD as a secondary drive). This way, you could dedicate your $700-800 solely for the cpu+mobo+RAM+a second 5850 (+maybe a better display)