(In Ciff Craven voice): Well actually Normy, that's not a garden valve. the Arrowhead valve discussed has an anti-siphon valve built in. The cheaper chinese ones I see have the same profile as the Arrowhead garden vales, but no built-in anti-siphon valve. I live on a hill, and run fertilizer through my sprinlkers occasionally so i am actually concerned about siphoning.
Oh yeah, a true 3/4 garden valve will flow twice as much water as a sill-****. The new 1/2 & 3/4 garden valves I've seen are really just 1/2 garden valve with the larger threads also. For flow, get a Arrowhead 3/4 Garden valve.