Sadly the GPU logic has likely failed (soldered on ATI Radeon HD 2600 GPU) or the high end offered a GPU module.
Or the LVDS cable connection to the display has become damaged or the LCD display its self.
* A used replacement logic board goes for $60 ~ 80 USD
* A used GPU board 2007 (2.6GHz and 2.8GHz models) $150
* A used display is about $80
* A used LVDS cable is about $12
Without doing more diagnostic work I’m only guessing.