Well, I've tried the 10.4.0 XNU kernel but for me, that one doesn't detect caches and disables my ability to use USB modems for internet. So far, this has been the only working kernel, which supports my phenom 955 fairly well.

I'm on 10.6.4 using this 10.3.0 kernel, it seems to work fine which suggests that Apple hasn't added anything major to the OS requiring new code. I suppose the only advantage I'd have with the 10.4.0 legacy kernel would be the on-the-fly CPUID patching. That's actually a neat feature.

I'll be checking this thread as well to see if anything comes along

All the best,
Tomi