Ok, just figured out how to make ASU patch .pkg files in my HD.
I don't know why, but ASU just can't patch MacOSXUpdCombo10.5.4.pkg.
Tried downloading FrontRow update and it patched successfuly, but somehow it doesn't detects cpuids in the update combo package.

I'm not 100% but the files I think you need to delete are mach_kernel, system.kext, AppleSMBIOS.kext, IONetworkingFamily.kext and all nvidia kexts.

Hopefully someone will be along to confirm what i've said

Re. ASU, you don't need to download the update again, in the link i gave you above it tells you how to patch it.

I've followed the instructions in that link but ASU simply doesn't detects cpuids in my MacOSXUpdCombo10.5.4.pkg.

Maybe because it doesn't have the option "Show package contents" on right-click and ASU can't read it? Don't know...

16:15:56 Downloading updates list ...
16:17:10 Updates list downloaded.
16:17:39 Preparing download ...
16:18:38 /private/var/folders/Rl/RlNpIvVBFKeOfbRBY3PSwE+++TI/-Caches-/com.apple.SoftwareUpdate/swcdn.apple.com/content/downloads/37/52/061-4731/RnMdwqhyLzvvWSVGvHRCGLKNwdHjhTz8HM created
16:18:40 Downloading iLife Support 8.3 ...
16:19:45 iLife Support 8.3 downloaded.
16:19:47 Moving packages to temporary folder ...
16:19:49 Extracting MacOSXUpdCombo10.5.4.pkg ...
16:20:19 MacOSXUpdCombo10.5.4.pkg extracted.
16:20:19 Detecting cpuids for MacOSXUpdCombo10.5.4.pkg ...
16:20:21 Done
16:20:22 No cpuids found !
16:20:22 No need to rebuild !


That's the log...
Can you understand the situation?

Why don't you use AMD patcher utility? I remember thats what i used to patch 10.5.3 before using ASU. Make a patching list, boot into single user mode and patch it from there.

I'm doing that.
Firstly must extract the .pkg content and patch it.
Then I must rebuild the package.
Right?

Yes, you'll have to rebuild it. I can't remember right now what i did exactly back then cause i was experimenting a lot. But as far as i can tell i think i updated using kalyway10.5.3comboupdate, i used to log into my osx installation on spare usb hh and install this update on my working osx partition. Then install kexts, applied patching, etc.

Right now you'll have more work to do with rebuilding the package. It all depends on what you are planning to do with it, keep it for later reinstalls, etc. I think you have somewhere to d'l pre-patched 10.5.4 update for AMD, i think i downloaded it and have it somewhere. Also you can directly install 10.5.4ComboUpdate you have right now. There is a howto from netkas and also some of them on insanely mac forum. I'd suggest you should back up all your working kexts, never forget to back up, and it'll be good to use spare partition, external usb drives for such things.
http://forum.insanelymac.com/index.php?showtopic=107340
Its all pretty well explained, 10.5.2 -> 10.5.3, but it will work for the latest update too. You'll have to install the kernel later, make list for batch patching, etc.. if you decide to go this way, boot into another leo installation and install it onto partition you wish from there; not to install it to installation you're logged on.

I've managed to update my system.
Here's what I did:

Extracted MacOSXUpdCombo10.5.4.pkg (using Pacifist)
Patched and decrypted all files
Deleted these files (from the extracted files):
- mach_kernel
- mach_kernel.ctfsys
- System.kext
- IONetworkingFamily.kext (if you use a patched one)
- AppleSMBios.kext
- AppleEFIRuntime.kext
- AppleIntelCPUPowerManagement
- AppleACPIPlatform.kext (if you don't use EFI)
- NVidia kexts

Rebuilt the package (using Packagemaker 2.1).
Installed patched update
Installed StageXNU kernel and System.kext
Cleaned Kext caches.
Rebooted with "-v -f" flags.

and...
Voila! xD