How much can I get for my computer?

I have an eMachines with 40GB IDE Hard Drive(had to downgrade on that because my old one busted) 512mb of RAM, a media reader including the most popular types of cards(SD, CF, etc) 5 USB ports, 335 Intel Celeron D processor, 2.80GHz, 256K L2 Cache, 533 Mhz FSB. I was hoping to get around $400 for it, I bought it for $800, but that was a few years ago. And I really want to upgrade.