hmm, i really don't think it's your heatsink. if your temps are reading correctly, then 52 degrees is too hot for case temp. that's the temp of my cpu and i run an 80mm fan at about 2000 rpms (pretty slow and quiet.) i also have four case fans (2 in and 2 out), but they are all 80mm stealth fans controlled by a baybus. they're all running at a quiet 1000 rpms. my case temps are in the upper 30s, low 40s.
be specific about ur ambient, because "low" is relative. i live in miami, and low temps down there run in the 15-20 range, but i go to school in upstate new york and low up here is -15.
it's easy to tell if your fans are working, just listen to them!
anyway, ur case temps seem to be too high. maybe you're fan filters are clogged w/ dust and your intake fans aren't running efficiently. also look into case clutter, i.e. cables, drives, etc.