I over clocked a Dell dude!
anyway
how about this
What I do is use Ebay and check similar models
then go on craigslist in your area and search your model number
then use those two price points
generally you get more on ebay then craigslist
on ebay you usually need to deal with paypal and shipping
with CL you can get cash and do a pickup or local delivery
I usually sell my towers on Craigslist
they both have pros and cons