You don't actually need to replace the display kexts with your old ones. Those are patched kexts from an older version of OSX. Also, you don't need to do anything with the bundle files, or at least I didn't on either the 10.5.6 or 10.5.7 update. You just need to patch the vanilla kexts to recognize the eeePC's video hardware. I've attached a shell script to this post that will patch the video kexts (AppleIntelIntegratedFramebuffer.kext and AppleIntelGMA950.kext) so that OSX will properly recognize the video hardware and Core Image/Quartz Extreme will work, and you will be able to set the proper screen resolution.
After you update to 10.5.7, open a terminal and cd to whatever directory you downloaded the script to and run "sudo sh ./VideoKextPatcher.sh". You should get the messages "Unknown AppleIntelIntegratedFramebuffer version (might work anyway)" and "Unknown AppleIntelGMA950 version (might work anyway)" (don't worry, it will work). Just to be safe, before you reboot, open up OSX86Tools, check repair permissions and clear extension cache, and run it. After you reboot, run OSX86Tools again and click the "Enable Quartz Extreme" button. You will need to do this every time you update OSX.
I have the 1000HEB (Best Buy version w/o BlueTooth, but identical video hardware), so I don't know about the BlueTooth kext. There's probably a similar patching method for it floating around. Replacing the kexts with the old pre-patched versions probably won't hurt anything, but it's probably better to just patch the current versions.
Also, just as a rule, it's a good idea to make a full backup before you update OSX or apply any security updates. Get Carbon Copy Cloner, which is free (well, it displays banner ads while it backs up, but they're unobtrusive and it's a small price to pay), and use it to back up your boot volume to a read-only disk image. That way, if you mess up and roach your system, you can boot the iDeneb installer, open Disk Utility, and restore the image to the drive. The iDeneb 10.5.6 and 10.5.7 updates both worked perfectly for me, but better safe than sorry.