I'm building a PC for AE for my job soon. My understanding of scratch disks is that when the machine runs out of ram, it puts that data that would normally go on the ram, on a separate drive. If I never ran out of ram, would it be necessary to have a drive specifically for scratch disk data? Or am I missing something? Thanks a lot in advance!
If you're using AE CS6 or CC, the cache system that it uses really makes use of separate, fast disk. On your new computer, get a solid state drive to use for AE's cache and there will be some significant speed increases in your workflow.
There is no "scratch disk" feature for After Effects.
There is a disk cache feature---which Szalam mentioned---and it helps with performance on any system.
There's a demonstration of the various cache features here: