What do you consider the definition of a "generation"? Is it based on hardware and/or software? Is it based on changing the way we use a phone? Is it based on new technology? Personally, I would say the Evo was the beginning of a new generation, as it was 1) the first 4g capable phone to the market and 2) the first phone to incorporate a front facing camera that allowed for video chat. It was a game changer, something that many other manufacturer's will try to emulate in the future.
The Droid X is a great phone, but even if it WERE the first phone to market with Froyo on it stock, I wouldn't say it was a new generation. One could argue that the addition of a full Flash experience (not lite or SkyfirE) may usher in a new generation, but I don't consider it to be so. The same goes for the first phone with 1.5 GHz, etc. These are merely speed improvements upon existing phones, nothing radically different. When we see a dual core processor, I would consider that a new generation, as it's something that will soon be adopted as the standard.