Intel GMA 500 driver for the Windows 8.1 is yet be released, but some experts have already been able to get it working. I own a Sony Vaio P with Intel Atom Z520 and GMA 500 GPU. Few days back, I install windows 8 on it. For obvious reason, graphics felt a lot choppier. This problem was fixed by downloading and installing modified drivers.
Download them and unzip them. Right click on setup and run it under windows 7 compatibility mode.
Then start Setup and proceed with the installation. Reboot and enjoy GMA 500 support in Windows 8.