Generally there isn't much of a difference, unless your mobo uses some config like Intel's 875/865 chipsets SATA implementation where it's a direct 150MB/s connection to the northbridge. If I'm not mistaken, the IDE bus bandwidth is shared with the PCI slots, but I can remember a diagram I saw some time ago where the IDE and PCI pipes were seperate.
Looks like I'm the one that needs questions answered...