Mac users, note that Folding@Home can't use GPUs at all. I considered Boot Camp but their docs (and Boinc's) seem focused on Nvidia and don't appear to have kept up on "ATI" (AMD) video card instructions.
"Compute units" is AMD terminology for a grouping of ALUs and stuff. Nvidia calls them "stream multiprocessors".
Example: Vega 64 has 64 CUs, each CU has 64 "shader processors", so there's 64*64 == 4096 "shader processors" total. This is the big part.