from your howto:
"# blacklist agp module
The above is correct only for the modules, not in the case when AGP(GART) is built by default in the kernel. AGPGART will prevent NvAGP from loading even if you set NvAGP "1"
In other words, this will work only with some, not all mobos.
This is why blacklisting does not work in general
Also read this:
and this about agpgart module: