Sampling rate in Wifi 802.11 -> 20 MHz enough with a BW of 20 MHz?
The radio-frequency signal of 802.11n can have a bandwidth of 20 MHz or 40 MHz. But even if it is 20 MHz, it can be sampled at 20 MSa/s if an I/Q demodulator is used: at the receiver the radio-frequency signal is downconverted to the baseband. The I/Q demodulator has two lowpass output signals (Inhphase/Quadrature or real/imaginary part) with bandwidth 10 MHz each. Two samplers with 20 MSa/s each are then sufficient to fulfill the sampling criterion.

There are other receiver architectures where the received signal is first downconverted to an intermediate frequency (for example 10 MHz) and is then sampled at a higher rate (for example 40 MSa/s). In this case only one sampler is necessary.

The guard interval has only a negligible influence on the signal bandwidth. It is not included in the "Fourier transform period $T$", because it is only added after the transform. The symbol duration after the transmitter IFFT is therefore
$$
T = N/f_\mathrm s = 64/(20\,\text{MHz}) = 3.2\,\mathrm{\mu s}
$$