I just upgraded today to the GTX 580 3GB from a GTX 285 1GB but I've seen no improvements at all while using Premiere Pro.
I did some tests with GPU Acceleration enabled and encoding times were for a 720p clip at 10 minutes on a 1920x1080 sequence were 17.34 for the GTX 285 and 17.31 for the GTX 580. (Both saved to a RAID 0)
I have the GPU and CUDA drivers updated to the latest.
Any ideas if I can speed up the process with the new card?