TGV (High Speed Train) is the best option, although it will take you more like 3 hours because the high-speed connection from Brussels (where there is a layover) to Amsterdam is not yet complete. Here is some advice on to book your tickets:
-Unless you want a Eurail/Inter-rail ticket and you want do a lot of traveling across Europe, don't individual tickets from the US. You get ripped off.
-Use the following site http://www.voyages-sncf.com - as a resource to select the ticket that you want. Unless you are a resident in France, you can't actually purchase the ticket. But it will help identify the ticket and the price. There are promotions all the time on tickets. Then, buy the ticket at any train station (you don't need to go to Gare du Nord, from where the train to Amsterdam leaves)
-You should be able to get prices as low and 100 Euros for a return voyage, about $120 - especially if you're under 26 and travel off-peak.
-If you want to travel really cheaply, consider taking a bus: you can travel for as low as 70 Euros and the expressway system in Europe is excellent, so it's usually fast. http://www.eurolines.fr is a reputable bus company.