Sampling frequency of modulated signal
Nyqvist-shannon sampling theorem  only says that your sampling frequency should be greater than twice the bandwidth of the signal and not twice the carrier frequency of the signal. Hence you can modulate your 20 MHz signal at any carrier frequency you need and still get back your original signal by sampling  at greater than 40 MHz. Hence you need not sample at 2GHZ for recovery of signal, you can do away with a sampling frequency greater than 40 MHz itself  For a more clear understanding refer to bandpass sampling.

http://en.wikipedia.org/wiki/Undersampling