If in US to compare Thalys prices and buy and have tickets in your hand i always recommend BETS (800-441-2387) for their expertise and competence - they work thru RailEurope but don't charge some of RE's fees and whoever you talk to will know what they're talking about from personal experience. Online try" www.thalys.com or www.voyages-sncf.com (French railways)
Thalys trains speed up to 186 mph on the Paris-Brussels trajectory then limp across Holland on old tracks at normal speeds - new Dutch highspeed line opens next year hopefully. At times Paris-amsterdam trains can be fully booked - in this case try to get the much more frequent Paris-Brussels trains then take the regular non-Thalys IC trains Brussels-Amsterdam that are nearly as quick but not as quick as Thalys as Thalys don't stop as often. 1st class on Thalys you get, in my experience, a fairly good meal with wine - 2nd class just a seat reservation. 2nd class is fine on this modern train.

Thank you SO MUCH!!!

Is it realistic to think that I could buy my 2nd class ticket the day before I want to take the train as I am site-seeing? I'll be taking the train on a Thursday. Thanks!

I think I am going with buying the tickets stateside with Budget Europe Travel. Talked to Byron there. Really helpful!!