first of all, by channel i meant an IDE "slot" on your mobo. a channel can support up to 2 IDE devices. generally a mobo will have two channels supported by the south bridge (kt133 and i815E), therefore you can connect up to 4 IDE devices. some boards, for example ABIT KT7, have an extra IDE controller, so you will find two more IDE slots on the board, which can allow another 4 devices. (a total of 8 together with the original IDE controller).
Now from what I learned, when you put an ATA100/66 device with ATA33 devices, everything will be downgraded to run at ATA33. Currently all CDROM, CDRW, ZIP, etc are all in ATA33. hard drive is the only thing that runs at ATA100/66. That's why I said don't put anything together with the ATA100/66 hard drive (unless that's another ATA100/66 hard drive).
I personally like to put the hard drive alone as the primary master (since I only have one hdd), and put all my CDROM, etc in the other channel.