I read a lot about how an external USB drive (the transfer speed to be more accurate) is the bottleneck when it comes to rendering.

So I am wondering... if, while rendering a project hosted on an external USB drive, my Task Manager shows both CPUs working 100% and out of 1GB of memory 1GB is used -- does this mean I have NO bottleneck?

It means that at the moment you measured these data, your bottleneck was CPU and memory. That does not mean that the USB drive will not be a bottleneck at a later stage. You likely have a multitude of bottlenecks. Whether that should bother you is a question only you can answer.