I also could use some fast SPI A/d converters that could be triggered to take the samples at the same time. So then where the signal came from could be triangulated.
my open source project is the pceeg at http://pceeg.sourceforge.net
It would be nice to use a pre existing open source hardware micro then my programmable EEG might become a shield board or add on or visa versa. Basicly the popularity of both could be worked on for more users. ;D 
:o
Resurrecting an old thread. I find your projects documentation confusing as the schematic is in an unknown format and the png graphics will not scale and do not seem to have reference designators.
If you want to do 8 channels and read them at the exact same time you will be somewhat disappointed. The chip you've chosen is very good but it uses a built in mux but there is a slight delay in switching between channels. There is no really good way to do this unless each adc also can store the readings and each be triggered to do a read. That would require something like 8 ATTINY + a single channel high precision ADC. Then there would still be a delay to read each of the ATTINY chips.