dunno nowdays really, when they came, i865 was the budget version, i875 was the "good" version.
Meaning i865 had no pat, limited dual channel possabilities and so on.. later (not that much later) some mobo maker made a "patch" that actually was atleast as effective as pat, and, dunno what happend to the "somewhat limited" dual channel possabilities, nowdays thats not a problem.
Usualy, nowdays, you dont need to look that hard on what chipset is used (of the two) but rather focus on the features you want.
B!