It doesn't seem to be AGP4x that's making the difference here, i840 smokes the rest and i820 can't score any better than BX at 750/100 MHz, although it comes with AGP4x versus the AGP2x of BX. It's got to be i840's superior memory bandwidth that makes it score so well. I guess the i840 is the new top platform for Quake 3 Arena players.
The picture is pretty much the same. i820 only scores at 133 MHz FSB, otherwise it performs almost identical to 440BX. i840 is the new Coppermine platform of choice for gamers.