Generation of N bit prime numbers -> what is the actual range?
In the context of RSA, when we say an "N bit prime", we mean that it's a prime in the range $[2^{n-1}, 2^n)$.

In addition, when we say an RSA key is an "N bit key", we mean that it's in the range $[2^{n-1}, 2^n)$.  What this means that if you pick two random $N/2$ bit primes, and multiply them together, you'll get an $N-1$ bit modulus about half the time.  To avoid this, one common practice is to select the primes from the range $(\sqrt{2}\cdot 2^{n/2-1}, 2^{n/2})$ - that way, when we multiply the two primes together, we'll always get an N-bit key.

If you're worried that restricting the primes to this range will make guessing them easier, well, that's actually not a concern.  If we're generating a 1024 bit key (which by today's standards is cutting it close), there are approximately $10^{151}$ primes in the range $(\sqrt{2}\cdot 2^{511}, 2^{512})$-- it's unlikely anyone will happen to guess either one.