A few questions before Overclocking

Ok so here are my specs,

Case: NZXT Phantom white (plastic Enthusiast)
Mobo: Gigabyte 990FXA-UD3
CPU: FX-4100
Cooler: Hyper 212+ (dual fan push and pull setup)
RAM: 16GB (4x4) 1866Mhz DDR3
GPU: EVGA 560Ti 2GB


First question, What raises heat the most in Overclocking; is it the HT speed, the Multiplier, or the Voltage (if you can put them in order from what produces the most to the least heat.

Second question, my current clock is 4.6GHz at 49C full load (8 hours of Prime)
my config is as follows
Multiplier: 21x
HT Multiplier: 220MHz
CPU VID: 1.3875V
NB VID: 1.2875V

would it be safe to say i could bump it up even more? and if so what would i need to bump up voltage, HT, or Multiplier

i dont want my temps to exceed 63+
any help is appreciated