Our MPEG-2 playback tests were done using CyberLink Power DVD version 7 with Video Acceleration enabled to get the best possible video quality. Playing back a documentary on the making of a movie (not going to say which nerdy movie it is), we measured with Throttle Watch over the course of a 20 minute documentary the average CPU usage as well as describe how the playback went, for a real-world summary.
MPEG2 playback is identical across the board here, we can see that there isn't much improvement in MPEG2 playback on the 2 setups, however the discrete graphics helps take a lot of the load of the system.
MPEG-4 playback was done with Media Player 11. We used the same MPEG-2 video that was encoded to DivX 6.4 with our regular test bed so we could have the same 20 minutes run. Again Throttle Watch was used to measure the CPU usage over the 20 minutes and give us a percentage.
MEPG4 playback improved on the EPIA-SN; even more so when the onboard graphics was shut off and the GTX was used.