PIIX4 vs. PIIX4E South Bridge questions

["John T. Croteau" <johnc_(_at_)_geekopolis_(_dot_)_com>]

How radically different is PIIX4 from the PIIX4E?  Judging by the .PDF
revision changes I downloaded from developer.intel.com, it does not look
like the changes are drastic.  However, I am no way close to understanding
chipset technology.
 
In a nutshells, what will happen if I try to install on a system with a
PIIX4E South Bridge and an Ultra/66 drive connected to it?  Will it work
just fine at /66 speed, will it fall back to a slower DMA mode, fall back
to non-DMA mode (PIO 2?), or will it simply not work at all?
 
If it won't work at all, what kind of information do I need to beg and
steal away from Intel so the developers can support this common chipset?
 
Thanks guys.
 
- JT