You'll probably not get much for the added display. I'd say $200 - $250. I assume it has the standard ATI Rage 128 PCI graphics card. Does it have a CD-ROM drive or a combo drive?
-- Mac Pro Dual 3.0 Dual Core, 4GB RAM, 150GB Raptor HD, 500GB HD, Superdrive, Radeon X1900XT, Bluetooth, Airport Exteme, Logitech Z-5500 5.1 Speakers, 30" ACD
-- PowerMac G5 Dual 2.5, 2.5GB RAM, 160GB HD, Superdrive, Radeon X800XT, Bluetooth, Airport Extreme
-- Macbook Pro 2.53 15", 4GB, 320GB HD, SuperDrive, Bluetooth, Airport Extreme