If just accessing the 60GB during normal operations (20GB for data store like .doc and backup), will this have performance impact?
If IDT is not supported by the 20GB, will my 60GB runs at ATA66 too (but still displayed by motherboard as ATA133)?
Simple answer to both questions, pcwlai: no. Use new 80-pin ATA100/133 cables, install the latest <A HREF="http://support.intel.com/support/chipsets/index.htm?iid..." target="_new">chipset drivers</A>, and you won't notice any performance problems that might occur by having the two devices on the same cable. It's also unlikely that you will notice a great deal of difference in the speed of the new drive even when accessing both devices at the same time.
Check the DMA settings on the drives (or the IDE controller channels, depending on the OS) in the Device Manager after the installation, and after installing the chipset drivers, to make sure that they are correct. If not, change them, and reboot.
Note: This is a <A HREF="http://support.intel.com/support/chipsets/inf/inf.htm" target="_new">link</A> that will tell you if the system needs the Intel Chipset Software Installation Utility, which normally should be installed <i>before</i> installing the Intel Application Accelerator.