vanilla? if you are not using vanilla gma950 kexts then you will get artifacts in memory hungry programs such as iTunes, firefox, msword.
if you have a core2duo or a coreduo then you need the TSCSync kext to get both cores, otherwise you need to use cpus=1.
if you have core2duo you need to force system into 32bit as well.
more details here and in noob's guide (who has the 6400 as well).
Well, yes and no. The GMA950 installer (enclosed in the package) ensures an artifact free environment. I always figured the TSCSync to be a bit buggy so it's excluded. The system boots to 32 bit unless you force it to boot to 64.
The guide you refer to is one of the worst to follow, the number of conditionals and unknowns are way to many. With this package (which i claim no credit for) your 6400-system is up and running within 20 minues. Take it or leave it, it's still here - working extremely well!