Nobody can say that the Xbox One is an underpowered machine. People will, however, argue whether or not it's as powerful as the PS4. It will probably remain a point of contention among fans throughout each console's lifecycle, but at least the Xbox One is doing what it can to pick up any perceived slack in the graphics department.
Microsoft's March Whitten was on Major Nelson's podcast today talking about everything Xbox One, and he said that the Xbox One has gotten a little more powerful since E3. The extra power comes from optimizing its graphics drivers to a little extra clock speed.
First up is optimization. It's been known since before the Xbox One was unveiled that Microsoft would be using Direct X 11 in its newest console. What we didn't know is that Microsoft has created its own graphics driver called the "mono driver" that strips Direct X of everything that doesn't fit into the Xbox One experience. From there, they added some more optimizations to make sure software can get everything it can out of the hardware.
Whitten goes on to claim that Microsoft's development partners are by and large now using the "mono driver." What this means is that games coming out for the Xbox One at or near launch should be better optimized than those games that came out near the Xbox 360 launch.
As for the hardware itself, Whitten says that they have been able to boost the GPU's clock speed from 800MHz to 853MHz. A 6.6 percent increase doesn't sound all that impressive, but every little bit helps. With the increase, developers will be able to push the hardware just a little harder.
So, what will all of this translate to when it comes to the actual games? We can't say for sure yet until actual multi-platform games start to hit store shelves, but Digital Foundry put together a comparison of two PCs running on Xbox One/PS4 target specs:[h/t: Kotaku]