Don't plan on carrying enough cash to last until you get your Alien Registration card. Even though Japan is relatively safe, I wouldn't want to carry several week's worth of spending money. Better to rely on your US bank account - Make sure you have an ATM card with the Cirrus or PLUS logo. You will be able to use it to withdraw cash at Post Offices, 7-Elevens, Citibanks (and a few other places - but probably not at a "normal" Japanese bank).