Modding the Categories Box

Hi,

My store is going to be based on a 3-levels-deep category tree. I want to modify the format of the Categories Box in the following way:

• No matter which category I'm in, all the Top level categories ("Level 1") should be listed at the top of the box.
• Below the list of "Level 1" categories: The name of the currently-selected "Level 1" category should be displayed.
• Then below that, all the "Level 2" categories (that reside in the currently-selected Top category) should be displayed, and each one of those should already have its own "Level 3" subcategories opened below it (without having to click).

Even if I'm currently browsing a "Level 3" subcategory, the box should display all of the above, according to the Top ("Level 1") category that this subcategory belongs to.
In other words, I want the Categories box to look something like this:

Categories

LEVEL 1 CATEGORY A
LEVEL 1 CATEGORY B
......
LEVEL 1 CATEGORY n

<Name of currently-selected level 1 category>
LEVEL 2 CATEGORY A:
---- Level 3 category A
---- Level 3 category B....
---- Level 3 category n
LEVEL 2 CATEGORY B:
---- Level 3 category A
---- Level 3 category B....
---- Level 3 category n
LEVEL 2 CATEGORY n:
---- Level 3 category A
---- Level 3 category B....
---- Level 3 category n

That's it, more or less. ;) Any php-wizards around here who can give me advice on how this might be accomplished? Help would be most appreciated!
(By the way, I'm using Zen Cart 1.3.0.1)

Thanks!