they are in the same tier that looks to be alphabetically listed as far as dual cores in the tier and not performance based. what the difference between two cpus in the same tier . . well is anyone's guess. as stated on that page, that hierarchy is for upgrading the cpu not necessary a performance measuring stick.
that list has fallen to quite a bit of scrutiny which after several years of reviewing cpus for gaming, it will be bound to happen that cpus get "misplaced". personally i believe several of those tiers ought to be split up at an "A" and "B" myself.
here is a bit of a comparison with an E8400 and G850:
http://www.anandtech.com/bench/Product/56?vs=404
the last four benches shows the G850 edging out the E8400 in 3 of 4.
looking at the numbers both have the same clock speed but the E8400 has 6megs of cache compared to the 3megs the G860 has. how or what difference the cache makes in gaming, i cannot truly tell you. though i do know the sandy bridge architecture is 15% more efficient that the C2D arch. but you could overclock the E8400 to compensate for that difference; i had one @3.8 without breaking a sweat with a better cooler.
i'll put it to you this way:
if you are looking to upgrade from a E8400 then wait until you can get an i3-2100 or better. if you are comparing between two platforms that you are going to buy then get the G860. the 775 socket is at its end of life and most motherboards are DDR2 which is very slow compared to DDR3. and also you would be able to upgrade to a much better cpu with a 1155 socket than any 775 socket cpu.