You're in the same boat as this poster. You're both dealing with older and dead technology. Quality parts for their day and age, but abandoned too soon by Intel.
While your system has a newer processor (and still a great one), it's limited by the power supply and graphics card. Anyone looking for a gaming system would need to replace both at a cost less than building their own, new system. Anyone looking for a just a basic computer for home use (web/e-mail/YouTube) could get one new for about $400.
I think you could probably get away with asking $350-$400 for your system.