Hi Grant,
I have been speaking to you about my mac pro with ate radeon hd 3870 pc card running el capitan on my post, I've just come across this post and noticed you have posted on it too, I see this changing on device id and wondering if i should be given this a go?
Find /System/Library/Extensions/ATI2000Controller.kext
(right click and select "Show Package Contents")
find the file info.plist within that kext.
Scroll down a ways until you get to a line called <key>IOPCIMatch</key>, after that should be a <string> line with lots of 0x0 numbers in it.
use some type of texteditor and add the device ID of your video card to the end of that <string>, an HD5450 should be 0x68F91002.
You can find the Device ID by installing the video card in an additional slot and going to "About This Mac" -> "More Info" -> "System Report" -> "Graphics Displays".
You should see Device ID 0x68f9.
Add 1002 to end of that value, so it reads 0x68F91002.
Reboot computer and OSX should now display it as ATI Radeon 5000 with correct memory amount. You can plug it into the main graphics slot. You will not get a white boot screen, but it will eventually show the desktop.
But should i try and change the device id in ATI2000Controller.kext? or could i use the AMD3000Controller.kext to enter the device id? thanks