If you use Tunebite digital dubbing, then the sound-card that you have, is no longer important, because the accelerated conversion is made through Tunebite's digital driver. What is important in getting the files converted at 4x without problems, is the cpu and RAM memory.

With the current configuration that you have, you might get 4x smoothly by trying the following:
-close all applications that you have running, and let Tunebite alone to convert
-open the Task manager and select for the process Tunebite, high priority level.

As for the configuration you are planning to build, Tunebite will run at 4x without any problems, with such cpu's. Then again, there is no need to buy such a high end sound-card as M-audio ones, only for Tunebite.