I want to buy 2 tickets Amsterdam-Buenos Aires in april 2005 for less
than 650 dollar each. Who knows good bookingsites?

try mobissimo.com

Mongolia

Not sure if it is the same there as in the U.S., but I found courier
flights are the cheapest, and we saved $1000 per ticket going to
Ecuador, by booking two seperate tickets. The best places to get cheap
international plane tickets seem to be online now. There is more
information at the site below, but it is oriented towards travelers in
the U.S.

http://www.EverythingAboutTravel.com/cheapinternationalplanetickets.html

I do not solicit for these people in any manner, but I have to tell
you that the cheapest way to get your travel is to get it early!  Once
availability ceases to exist, so does your opportunity to get a good
deal on it!
Call the number listed on bestfares.com and call quickly!

Your $5 is pretty safe since even flights from Florida to Buenos Aires
would cost $700 plus!
Just as well you are not plugging bestfares ginaenet-ga,since they are
$400 per ticket more expensive than some others to Argentina.
You are right,mochilero-ga,that courier flights are very cheap,but it
is best if you can be flexible on dates and trying to get one ticket
is a lot easier to get than two,around the right time,unless it is to
a popular destination,which I fear Buenos Aires is not.And especially
not from Holland.
Although web prices are often very good, the best prices are obtained
by calling a consolidator [they are not allowed to advertise their
best prices].I
do not know of any Dutch consolidators though there is one in Frankfurt that
specialises in flights to South America,and there are many more in
England where I live.
You could also try www.priceline.com,which is an American site where you can 
post a price that you are prepared to pay,and see if any company will
accept your price.You will still need to get to the States
though.Travelling early in the week usually is cheaper than flying
over the weekend when airlines are busier.