China plus Hong Kong held about $200 billion in treasury securities as of Dec '03. Japan held about $545 billion in the same month.
The lesson is: if Japan decides to let the Yen float, America enters a recession. However, Japan's economy is so weak that doesn't seem likely for the forseeable future.
As you say, and amount currently held by China and Hong Kong is a little over $210 billion. The total amount, on the grand total line of your link, is the $1.5 trillion dollar amount I was referring to.
Although I think China represents a greater strategic threat than other countries, the total amount of debt (all of which must be serviced) is important IMO.