Most motherboards come with 2 ide channels. Each ide channel allows you to use 2 ide devices which means your basically limited to 4 ide devices.
A few motherboards like Asus's A7V come with an additional ide controller chip which means the motherboard has 4 ide channels. This in turn means you can then use 8 ide devices.
My suggestion is you get a promise ultra 66 ide controller card:
http://www.promise.com/Products/idecards/u66.htm
or Promise Ultra 100 ide controller card
http://www.promise.com/Products/idecards/u100.htm