Hello,
I just got a Nintendo DSi XL, and I must say that the screen is excellent, and I really think that it has a chance to compete as an ereader if Nintendo chooses to go in that direction.
If you want to make it a more versatile ereader now, you will need a Flash Card, a micro sd card to store your content, and some software. You can buy a cheap DSTTi card (make sure you get an i version), and then you are free to use legal, homebrew applications.
On the flash card I reccommend DSLibris and DSReader. DSLibris is my favorite and supports epubs and XHTML formats. DSReader supports .txt format. Both allow you to adjust the font style and size, and also other screen options. However, DSLibris allows you to hold the DS like a book (why I like it the best), while DSreader only works with the standard orientation. They are both great, easy to use programs, and I have them both installed. There are other options out there, but those are the two that I find to be the best.
If you get one, let me know and I will help you set it up
