I was just wondering, why are we using MHz when we rate the speed of RAM and not something like GBps or Gbps? Is it even the same thing? And if so what speed in GBps would 1000MHz be?
They are two very different things. GBps or Gbps are measurements of bandwidth. Hertz is a unit of frequency 1 hertz is one cycle per second. You can find the effective bandwith of your memory which may correlate a bit with its frequency but there are many other factors involved.
Frequency is only one of factors in determining true speed or bandwidth of given RAM. Frequency is important but Cas and other latency counts too, even quality of chips and controller. As same RAM will depend on CPU and MB (chipset) it would be relevant only with given setup but not other combinations.