Disassembled is usually best when the PC is a few years old - an out of date PC will only be bought by somebody needing a basic computer for a small amount of money, whereas the parts will be bought up by people with older computers wanting to keep them going (e.g. a few months back I sold a couple of AGP graphics cards for £20-£30 each).
If the PC is much newer, as here, it's best to sell it whole. For 'nearly new' items people look at the retail price and knock some money off to account for the item being slightly used and not having the full retailer protection if things go wrong. So the best you can hope for selling disassembled is the sum of the retail parts (if you sell them all) less some percentage, whereas if you sell the PC whole you're looking at the retail price of a similar spec PC, less some percentage, the key being that the retail PC will be always be priced at more than the sum of its parts. Not to mention too that you get rid of the whole lot in one go.