My former PC had a Intel Core 2 Duo (1.66 GHz) processor which took about 1 hour to convert a 45-minute DVR-MS file to WMV format for my Zune (using DVRMSToolbox with FFMPEG and Windows Media Encoder 9). I recently upgraded to a Intel Quad 2 Core Processor (2.33 GHz) which now can convert that same video file in only 20 minutes! So going from 1 hour to 20 minutes is a great speed up, with all four core processor churning away at the conversion.
So, upgrading to a Quad processor is a definite benefit if you want your video files to convert faster! 🙂