At an average for high end tasks how many times faster is the core 2 duo e6600?

The AMD Sempron will be at its bottleneck. AMD Sempron processor was not meant for those kind of work load. Maybe net browsing, Word processing, Listening and Ripping MP3, Watching DVD movies, but not video encoding, maybe a few game will run on it. Sempron is limit in its Cashe memory were the Core2Duo have a lot of Cashe memory built into the processor. The Core2Duo e6600 will be enough to play games and some video encoding. For full video encoding without the lag, I recommend the Core I7 processor. A little pricey but will do the job.