I run FOG in a VM with 4 cores assigned, and 4 gigs of ram assigned.
I never see the CPU utilization go above 12% ish.
I’ll add that I had accidentally assigned 1 core recently on a new build (just forgot to set that) and ended up installing 32 bit Fedora 21 server. Just the web functionality was so slow, it was unacceptable. I ended up rebuilding after assigning the normal 4 cores, and all is well.
That’s just my experience.