These are the second Game Ready drivers from Nvidia in a rather short time so do not expect a lot of improvements, other than game optimizations for a couple of games including the newly released Bethesda Fallout 4 game.
In addition to the released Fallout 4 game, the new Geforce 358.91 WHQL Game Ready drivers bring optimizations for the upcoming EA/DICE Star Wars: Battlefront game and Blizzard's StarCraft II: Legacy of the Void. According to Nvidia's post, the new Geforce 358.91 WHQL Game Ready drivers include all bug fixes and performance optimizations as well as SLI profiles and Geforce Experience settings for these games.
Nvidia has also published a blog post saying that the the Geforce GTX 970 is enough if you plan to play Fallout 4 game at high-level of detail and with Nvidia Volumetric Lighting effects enabled, all on 1920x1080 resolution. If you want to push the resolution to 2560x1440 with the same amount of details, you will need the Geforce GTX 980 Ti graphics card, according to Nvidia.
Just a few days ago, Nvidia released Geforce 358.87 WHQL Game Ready drivers which brought optimizations for Call of Duty: Black Ops III as well as a few other games.
New Geforce 358.91 WHQL Game Ready drivers support all Geforce graphics cards since Geforce 400 series and are available for Windows Vista, Windows 8/8.1 and Windows 10 in both 32-bit and 64-bit versions.
As always, you can download the new driver at Nvidia's driver support page.