If you have some spare cash you don't mind being tied up then I say buy at least 500. As the yen is currently dropping, currently 116 from it's high of 121 a month ago, it's possible the yen could get stronger and dollar weaker. Problem is we don't know what the future will hold. The yen weakening over the past few months has been incredible, considering what it has been for the past few years, but has reached it's peak for now. Future meetings/reports within the US & Japanese govt will affect the Yen & USD. With oil being as low as it is has been helping to make it stable and oil seems like it's going lower.
So to summarize, buy now if you don't mind having some money tied up. I usually charge for most of my shopping expenses and only need yen for dining at most places around Tokyo. At TDR I charge everything since credit cards are accepted pretty much everywhere, though you should still bring some yen to TDR in case their credit card machine goes down or your credit card transaction gets blocked.