Buses run infrequently and are privatized so it's extremely difficult to document. Taking the subway to Giza and connecting by mini-bus is much more difficult then catching the No 355/357 behind the Egyptian Museum. It's an "unofficial" stop, costs two Egyptian pounds, is air conditioned and will take you right to the base of the pyramids.
Things to remember, you have to "hail" buses in Egypt for the most part. Behind the museum a local will normally do it for you and you can just piggy back on their quick acting. Bus numbers are in Arabic, mini-bus or regular. My girlfriend always tells people when looking for that bus, Arabic is backwards "7" "0" "V". Good luck and have fun. Egyptians are friendly and will help you if you ask, just don't let them lead you anywhere...you'll see what I mean ;-) All harmless fun though!