sampling frequency of GPS
C/A code (the main civilian channel as of now) has a main lobe of around 2MHz and depending on how much processing you want to do, it is normally sampled in the low MHz range, which you are in. (you could pull it off with 2-5MHz).

Do know that the signal must be mixed down to a lower frequency before you sample it. The original signal is at 1.57GHz.