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.