NZXT do a nice range of hardware monitoring addons for the PC, bay slots or free standing.
Keyboard wise I'd suggest a G15 or G19, i've got a G19 and Aida64 which frankly is amazing for hardware monitoring but costs a fair whack (was a present or i'd never had done it).
There are plenty of software addons, motherboard monitor etc do it but most won't display over the top of a game in full screen mode where as the NZXT and keyboards obviously get around that. However the software monitoring applications can usually log as you play so you could check it retrospectively.