The highest frequency signal that you can reconstruct without artifacts is half the sampling frequency according to Mr Nyquist.

I don't know if anyone mentioned this, but that's true if what you are measuring is sine waves. If you want to measure and see a 12MHz square wave (for example --or for that matter any other non pure sine wave), you're gonna need a lot more than 24MHz. As I'm sure we all recall, a square wave is made up of tons of higher order odd harmonics...