You can submit 64-bit apps for iOS 7 today that take advantage of the power of iPhone 5s. Xcode can build your app with both 32-bit and 64-bit binaries included so it works across all devices running iOS 7. If you wish to continue to support iOS 6 then you will need to build for 32-bit only. Next month we will be making changes that will allow you create a single app binary that supports 32-bit on iOS 6, as well as 32-bit and 64-bit on iOS 7.
The iPhone 5s is the first smartphone to utilize a 64-bit processor. Apple is promising up to twice the CPU and graphics performance for almost everything you do on the device. You can take a look at some early benchmarks here. More benchmarks will be posted once the device is released on September 20th.