It's possible to get a motherboard with SCSI built in... but it's not common. Personally, I'd recommend if you really want SCSI, you get a SCSI card rather than a mobo in case you ever upgrade. That way you don't have to ever look for a mobo that specifically supports SCSI later on.
I think you understand that there's up to 16 connectors on a SCSI cable, but I just want to be clear that there tends to be only one connector on the mobo or PCI card. You can get multiple "channels" (channels are connectors, essentially), but you certainly never have 16 connectors on the board
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IDE hard drives can take full advantage of ATA-100 or better if (and only if) there's only one device on the cable. I'm sure someone else could explain that better than I could - I'll have to admit that I've pretty much taken it for granted.
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Primary
CPU: Athlon 64 X2 4400+ Mobo: Biostar N4SLI-A9 RAM: 2G Crucial (DDR400) Video: eVGA GeForce 7900 GTX (512M) Audio: HDA X-Mystique HD(s): Maxtor 300G SATA2, Samsung 400G SATA2 OS(s): WinXP x64 Pro, Vista x32 Ultimate, Gentoo x64 Monitor(s): Primary - 19" Flat Panel (1280x1024) Secondary - 19" Flat Panel (1280x1024) Tertiary - Zenith 42" Plasma TV (1024x768 res)
Many other machines... sig too short