In order to take advantage of Gala's hardware decoding, users will be required to have Mac OS X 10.6.3 running on a Mac with one of the following graphics cards: NVIDIA GeForce 9400M, GeForce 320M or GeForce GT 330M. Those Macs include MacBooks shipped after January 21st, 2009, Mac Minis shipped after March 3rd, 2009, MacBook Pros shipped after October 14th, 2008, and iMacs that shipped after the first quarter of 2009. Mac Pros are not supported at this time.
The Gala preview is available for download now. Until the release is finalized, users will see a small white square in the upper left corner of the Flash video while video is hardware decoding. I've only played around with it for a few minutes, but I can happily report that I've noticed huge improvements in the playback of HD Flash video on YouTube on my 2009 MacBook Pro.