As Jim says above, the crossing at Le Perthus - La Jonquera is fairly gentle. The coast route Port Bou - Cerbere is S-bendy and steeper - but not a long climb.
Choice depends mostly on whether you're coast hugging or taking a directer inland route to Barcelona.
Another way to get down from Amsterdam to Barcelona would be to follow the rivers Rhein, (Mosel) and Rhone to the Mediterranean...
Last edited by imi; 05-23-11 at 11:05 AM.