Have a lookit this:

Nyquist theorem. The Nyquist theorem states that if a signal V(t) does not contain frequencies higher than fs/2 (where fs = 1/Ts), then it can be fully recovered from its sampled values V( nTs) at discrete times tn = nTs ...

http://www.digital-recordings.com/publ/pubrec.html

I want to move to theory 'cause everything seems to work there. In reality the higher than x2 the sample rate is the easier it is to implement. I suspect a x2 system might actually be impossible to build, but I'm not enough of a math genius to prove it.