That's why some of the cpu engineers were focused on building a CPU with multiple cores that can work as a single one, rather than the opposite that we're used too.
That's because using more than one core requires the software to be adapted to that kind of processing. Also there are some algorythms that simply CANNOT be multi threaded due to their mathemathical nature.
As of software, like the preceeding post said - the speed will be likely limited to your media (HDD, optic drive, streaming, whatever) throughput.
As of encoding (not ripping) i recommend one of Xilisoft's software. It's not free, and acts poorly as of mkv ripping, but does a great job for other formats. You can also try using a free "SUPER" (yes, that's the name) software, though if you have little experience in the filed, using it may prove difficult, if not impossible.
Good luck mate.