Apple has released a Snow Leopard graphics update to address some graphical issues introduced in Apple's 10.6.4 update for OS X. Specifically, the update fixes frame rate issues with Valve's games Portal and Team Fortress 2, and it fixes a problem that may have caused Aperture 3 or StarCraft II to crash or become unresponsive. The update also resolves graphics corruption issues brought on by disconnecting and reconnecting external displays while a Mac is running.
The graphical issues with Valve's games following the 10.6.4 update received a fair bit of publicity. Fortunately, this update should resolve those issues. Let us know in the comments how it goes for you.
Snow Leopard Graphics Update is 69 MB and is available in Software Update or via direct download here.