I am... confused? My old PC has an X1800 XT, which is significantly more recent than that, and the Linux driver hasn't been updated since 2009. I don't think that driver even works with modern X.Org or Linux kernels. Could someone tell me if I'm missing something? Would be very happy to be able to run this card at full resolution, instead of having to use a generic VGA driver.
Your link is for the proprietary driver, this link is for the open source driver (not AMD themselves providing updates). X1800 XT is R500 so should be supported by the open radeon driver just fine, including output and modesetting https://www.x.org/wiki/RadeonFeature/
https://www.amd.com/en/support/graphics/legacy-graphics/ati-...