Please, just please, don't get a CPU just for Citra. This emulator is just not optimized enough now. The thing is that Citra will get optimized more, so users could play with decent framerates on older systems with OpenGL 3.3 .
Or if you really want to buy a CPU (but not just for Citra), buy the i3-7100. It has a better single core performance than most of today's CPUs. Has Hyper-Threading (2 cores, 4 threads) and could be paired with a decent GPU.
EDIT: Kinda ninja'd by @RavenHome, because they didn't specifically said that they don't have the 3DS. Why should they buy 2 consoles?