As I understand it:
i875 can support ECC RAM, has intel's PAT (Performance Accelleration Technology) which supposedly helps performance.
i865 is lacking those two features, but seeing as:
a)ECC RAM is expensive, and of no real use for home users anyway.
b)many mobo manufacturers have found sneaky ways of implementing PAT on their 865-based mobos. due to the licencing issues they have to call it something else, but it's basically the same thing.
I would say there is no reason for a home user to buy an i875 Board over an i865 board, given the extra cash could be better spent elsewhere.
I've not got any experience with i865 or i875 boards myself, but I've seen countless recommendations for the Abit IS7 in these forums, and I can't recall a single complaint about it - certainly if I was buying a P4 system now that would be my motherboard choice.
---
Epox 8RDA+ rev1.1 w/ Custom NB HS
XP1700+ @205x11 (~2.26Ghz), 1.575Vcore
2x256Mb Corsair PC3200LL 2-2-2-4
Sapphire 9800Pro 420/744