Once upon a time, Microsoft had to write off $900 million betting an Arm-based Nvidia chip could power its first flagship Windows portable, the original Microsoft Surface . But today, it’s trying again. Microsoft and Nvidia have just announced the Surface Laptop Ultra, a computer with a new Arm-based Nvidia chip at its core. There’s a lot we don’t know about the 15-inch Surface Laptop Ultra, such as its final specs or the foggiest idea of what it might cost. But Microsoft is promising it’s the most powerful Surface, period: “This is the most powerful thing we’ve ever made,” Microsoft Surface boss Andrew Hill replies, when we ask how it stacks up. It features Nvidia’s new RTX Spark “superchip,” which is roughly the same processor the company already sold in its DGX Spark mini-PC for AI developers , but now optimized to work with Windows 11 instead.…