Hi Dave,
If you're still up to the challenge, here's a quick guide to get you started with your HD5450...
• First boot with the kernel flag "GraphicsEnabler=No" (without the quote marks), this should at least get you a display so that you can make further edits.
• If not disable your discrete card in the BIOS or simply unplug it for now and reboot.
• Once you get a desktop you can make a few tweaks and hopefully get basic support for your HD5450
• Now navigate to the Macintosh HD/System/Library/Extensions folder.
• Here you need to find and copy the following two .kext files to your desktop for editing ---> AMDRadeonAccelerator.kext & ATI5000Controller.kext
• Now Right-click (or Ctrl + Click) on the AMDRadeonAccelerator.kext that's on your desktop and select "Show Package Contents"
• In the new window open the Contents folder and open the "Info.plist" file in TextEdit.app
• There's A LOT of text in the document. Don't freak out, we're literally adding in one PCI ID for your card.
• Do a text search for "Cedar" (this is the family to which your HD5450 card belongs). There's only two results and they're both exactly where we want to be.
• Look down the list of keys and strings and find the line that says <string>0x68E01002</string> (It'll be right below <key>IOPCIMatch</key>)
• Now add in this... 0x68F91002 (this is the PCI ID for your card and almost all HD5450 cards) after 0x68E1002. The line should now look like this <string>0x68E01002 0x68F91002</string>
• Save the file and close.
• Now Right-click (or Ctrl + Click) on the ATI5000Controller.kext that's on your desktop and select "Show Package Contents"
• As you did with the previous Kext file, navigate to and open the "Info.plist" file in TextEdit.app
• About half way down the document you'll see a string with a long line of PCI IDs (now that you know what they look like)
• As you did before simply add your same PCI ID (ie. 0x68F91002) to the end of the list.
• Save the file and close. You're done editing! Yay!
• This is entirely optional, but you may want to select both of these files, Right-Click them and assign a Label Colour of some sort before installing them. This will help you identify them later in the Extensions folder as having been edited in some way.
• Now you need to download a Kext installer/utility/app of some sort.
Kext Wizard is my favourite as it gives you the option to make back-ups of any original kexts that you you're replacing (just incase things go wonky and you need to put the originals back in place). It also has a tool to rebuild your Kext Cache.
• Cool! Use Kext Wizard to install your two modified Kext files. It's a drag and drop affair. Just remember to tick the "Backs Kexts..." box and then hit install.
• Now Execute the "Rebuild Cache" script under "Maintenance"
• You should now be ready to reboot
• With your HD5450 card enabled/plugged in, Boot with the following kernel flags "GraphicsEnabler=Yes AtiConfig=Null -f" (without the quote marks)
This enables Chimera/Chameleon to identify your card; assign the generic framebuffer personality (ie. RadeonFrameBuffer); and forces all Kexts (including the ones you've just edited) to load.
Whew! Now with a bit of luck you should now have a mostly functioning display via your HD5450 card and multiple display support.
PLEASE NOTE: At this point your card will only be mostly functioning...
DVD Player and Chess WILL crash when you launch them and you might not have 100% native acceleration (but hardly noticeable). If this isn't too much of a deal breaker for you, you can continue like this until you want to delve a little deeper into getting a 100% working card with native acceleration and HMDI Audio and no crashing apps etc...
I hope this helps and good luck!!!