1500. Max. I wouldnt bring less than 1000 if you can. Why? Because youre not JUST going to sight-see; trust me. You will stumble into a bookstore, toy store, gadget store, video store, etc. etc. - and SOMETHING is going to jump out at you, and you are going to WANT that something.
When I went for a week to just do "sightseeing" and what not - I brought about 800 and while I wasnt struggling - there was so much that I wanted to take back with me but couldnt, because of the money situation. Plus, you may want to get out to Yokohama, Hakone and surrounding areas - as well as drop bones at some of the parks located nearby. All in all - you want to be able to do all this w/o having to worry about "do I have enough money" to accomplish this.