how to increase the voltage of a digital signal?
For this very same purpose I have used the BSS138 mosfet with 10k pull-ups. I was able achieve 250k reliable communication with a micro running at 16MHz. A step up to 500kHz will surely work, I just could not reliably test this with a slow micro.

Also, the term for what you are describing is known as Level Shifting;



You can pick a board up using these same mosfets at Spark Fun.