well the killer is that you can go on dell's website and get a new computer (with decent components) and a new 20" monitor for 400 dollars nowadays so I wouldnt get your hopes too high.
I recently sold my system on ebay:
asus maximus formula x38 mobo
2 gb corsair ram (with ocz ram cooler)
WD 160gb harddrive
ati HD 3870 (before all the pricecuts)
a full dangerden watercooling loop (pump, block, connections..the works all in like new condition)
included a copy of win vista ultimate (upgrade dvd)
&games (crysis, C&C tiberium, Half life 2, Fear & Fear extraction point)
...all for a whoping 600 dollars.
you should deff. take the time to part it out as I sold all mine together and got raped. (on ebay)
It all depends on where you try to sell it. You may be able to get away with that, selling it on Ebay or some other local classified section. Basically, if you find someone uneducated in PCs, you could sell it for what you want.