I can second going to the Arenberg, it's a great time to be had; lots of crazy partying people and the atmosphere is just awesome! After that try to find a section of cobbles to view on the way to the finish or go straight to the finish in Roubaix. You can get into the velodrome free of charge and there is a big screen to watch the race.
Do be careful not to have any valuables in the car as the area around the velodrome is quite bad. We had our GPS stolen last year (it was tucked away in the glove box, but they still broke into the car as they must have figured it was there by the holder still on the window).
Be sure to get the Arenberg early and park the car pointing in the right direction to get out to make leaving as easy as possible.
If going to the races on your own is a bit of a hassle I offer guided day trips for the day of Paris-Roubaix (and the Tour of Flanders) through my
bicycle tour company, Chainring Tours. The trip starts from Oudenaarde, Belgium; includes seeing the races in three spots (Arenberg, another cobble section and the finish). We include a Cheese and Wine lunch buffet at the Arenberg Forest and dinner that night in Oudenaarde. The day-viewing option is 175 euros a per person.
You can see photos from our trip to the Paris-Roubaix here:
http://www.facebook.com/photo.php?pid=2630373&id=99237268786
We had a lot of people very envious of our cheese and wine spread.
I hope that helps give you some direction on how to go about seeing Paris-Roubaix.