Using HandBrake we converted a 560MB mkv file to the mp4 format and measured the frame rate each processor was able to achieve. The Core i5 661 did quite well with 45fps, making it slightly faster than the 44fps of the Core 2 Quad Q6600.
The Core 2 Duo E7400 managed just 26fps, while the Phenom II X2 550 was only slightly faster with 28fps.
The x264 HD Benchmark 3.0 is used to measure the performance of a 2-pass 1280x720 encoding, again displaying the results in a frame per second format. The Core i5 661 was able to match the quad-core Athlon II X4 620, which made it 9% faster than the old Core 2 Quad Q6600.
Using TMPGEnc we converted a 715MB avi file to the mpg format and measured the time it took for each processor to complete this task. The Core i5 661 was very impressive, roughly matching the Phenom II X4 945, making it one of the fastest processors tested.