A good way to determine the value of the parts would be to find them online either new or used and deduct an appropriate amount (if you're comparing to new parts). You also need a case and power supply.
Operating system could be worth about $70-80
Video card maybe $40-50
Memory $30
CPU $45-50
One problem with assessing used-part monetary value based on sites such as eBay is that the prices you see are generally those that someone wants for the item, not per se what people are willing to pay for it. With new home-oriented OEM systems being sold for only a few hundred dollars including peripherals a used one isn't going to be worth much.
Asus M3A78-CM -- asking $30 to $80 but no-one is buying at this price, no bids.
Vista Home Basic 64-bit -- if this is retail you can use it for a new build and upgrade it to Win 7 64-bit. If it is OEM its license stays with the old motherboard. Not too many want a copy of Vista.
AMD Athelon(tm) 7750 -- people seem to be buying these for about $25
NVIVIA GeForce 9500 GT -- buying at about $30
4 GB of used DDR 800 RAM goes for about $15
Hitachi 320GB HDD -- $15 to $20
PSU -- used OEMs are practically worthless -- $5, not worth shipping them
Case -- aboout $15
ODD -- CD/DVD RW -- about $10 if you can even sell it (they are $20 brand new).
Conclusion: Don't expect much much more than $200, even if you part it out and find buyers.
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