Hi there
Found this info at Extreme Sytems and thought it made things pretty clear.
The important numbers:
First digit is the generation in the series (higher = newer)
Second digit is the level of performance (higher = enthusiast, lower = budget)
Third digit indicates updates, revisions, or new features (higher = new features since the previous one)
The important letters:
F = Fast or "F'ing awesome," basically a good overclocker
G = has onboard graphics
X = CrossFire capable
The 940 is supported on the GA-MA790X-UD3P out of the box and with BIOS updates it will also support the currently available Hex Cores. As a stepping stone it should serve you well.