Use 2.5.70 kernel
I'm pretty sure that you wont get I865/875 AGP support from either the 2.4 or nvagp.
I've downloaded a copy of kernel 2.5.70 with patch-2.5.70-bk19.bz2 from the snapshots directory and have found that their is support for I865/875 in the Intel AGP module.
Your best bet is to try out the Intel AGP in 2.5.70 and go from there ..
If I get the chance, I'll try that out today and respond ..