Component Solution: Tree Cachcing, good or bad.

i am writing an object that will query db retrieve all data it needs and stored into a collection, example: mailbox object, has a property messages as a collection of class Message, that represent each message. It works very nice and fast. but the problem is that I've only working with a small number of data records...Would it still be practical if the returned record get larger and larger with today's memory availabitity...(Think of my program behavior like MSXMLDOM, load everything into a tree)
So, after load all data into the tree (mulitlevel of collections of objects), I release all db connection. Theoratically, it work well, very n-tiered oriented. my concern is that how it will effect the performance when there are millions of records return (i only expect around 100,000 items in a busy system at anytime).