You might also need this if your PS2 doesn't have an ethernet port.
Sony PlayStation 2 Online Adaptor Modem reviews - CNET Reviews
I have a Xbox 360.
Not sure what PS2 has to offer, If you are going to put a few hours a week playing against friends, The money is probably worth it.
Not sure if there are any other benefits. I hate playing with random people online, Especially Madden. I prefer to play a realistic game with complete play calling and punting on 4th down.
If you play online, you will mostly see guys just doing whatever they can to improve their rank. They couldn't careless about just having fun with the game itself.
A PS2 player should jump in and give you a better Idea of what PS2 Online offers, I'm obviously bitter about my online Madden experience.