The i810 and i810E chipsets used an integrated i740 video, while the i815/i815E used the i752 video.
The i752 is supposed to perform a little better than the i740, although I don't think anyone has gone out to quantify the difference. The RAMDAC on the i752 is 230MHz, as opposed to 203MHz on the i740.
Both the i740 and the i752 are actually decent performers when you use the DC100 on the i810 and i810E chipsets, and the external 4MB AIMM card for the i815/i815E. While the speed is not the fastest (think Riva128 speed), image quality is generally excellent.
Note that the i810 and i810E are different chipsets in terms of the "North Bridge", while the i815 and i815E are the same chipset with a different ICH.