Both are $50 at Microcenter with a motherboard. Which would you pick for a budget build? Why?
Pentium G645 2.9GHz 2 cores,2 threads (Sandy Bridge) 65w
FX-4100 3.6GHz 4 cores, 2 modules (Bulldozer) 95w
I say pick the Pentium G645 because the FX-4100 is beaten in almost all games even by the Pentium G630 (2.7GHz).
For example, Skyrim benchmarks: http://www.tomshardware.com/reviews/gaming-fx-pentium-apu-benchmark,3120-3.html