Most of my use is under Windows/Bootcamp, so for a quick test, I timed how long the 5870 and the 7970 took to thumbnail ~150 home movie files (720p/1080p AVC HD files) when opening the folder in Windows Explorer - both setups took the exact same time..
Windows makes no use of the video card to generate thumbnails so this is no surprise... the bottleneck here will be your disk and/or CPU.