Originally Posted by shunx
Found this post on VideoLAN: "Most encoders and decoders are nor dual core compliant at this time so dual core processing for Media files is not very helpful. A dual core processor will also need to be 3 GHz per core or higher to work properly [with 1080p]."
Problem is that clearly only refers to Pentium4 dualcores. A Core2Duo at 2ghz is maybe 20% faster than a P4D at 3ghz, and an Athlon 2ghz is about the same as the P4 3ghz (all very approx, but you get the idea).
Even more important is what type of material you're decoding, the bitrate, and how well your graphics card accelerates that format. For example, MPEG2 1080i at 6mbit may only take 10% on some setups, because the graphics card accelerates it easily. However, the same resolution in 35mbit h264 with full encoding options can max both cores of a 3ghz Core 2 Duo. In other words: resolution alone is pretty useless as a prediction of how hard it will stress your hardware.
Personally, I find the major use for dualcore in decoding is with h264, for the simple reason that everything else doesn't stress the processor enough. For that, PowerDVD and coreavc both use dualcore perfectly, and you almost double the performance with dual over single cores (dual core use usually has quite some overheads, but for decoding it's pretty efficient).