Will the SIM card I bought in Germany last fall work in Amsterdam? Or will I need to purchase a new SIM card specifically for the Netherlands?
This is not the definitive answer to your question since it is not based on first hand experience, but I think it represents the situation. When I was planning to visit Europe last September I asked about European SIM cards. Everything on the internet indicated that a SIM card purchased in an EU country is supposed to work in all other EU nations without roaming fee so you should be ok. Unfortunately I was going to Russia and had to purchase a SIM card that was not valid in EU countries.
Is the SIM card you bought still active now? When will you be in Amsterdam to use it? What company did you buy it from? They don't stay active forever - it varies by country and even by company I believe.
And the other question is: how can you buy "top-up" credit for it without going to Germany again? Some companies let you top up online without going back to the original country; others don't allow it. It varies.
This might help with more info about various mobile companies in Germany:
For example, the Dutch Vodafone SIM card I bought on eBay last year is supposed to stay active for a year after I last added credit to it; I am planning to use it this spring in Europe. I know I can add credit to it online with my US credit card as well.
If your SIM is still active when you get to Amsterdam? SHOULD work. EU roaming fees have mostly been eliminated, but not all SIM card plans allow roaming at all. Again - it varies by the company and type of SIM you bought.
"Everything on the internet indicated that a SIM card purchased in an EU country is supposed to work in all other EU nations without roaming fee so you should be ok. "
To clarify, Andrew is right. A SIM bought in one EU country will not have extra fees for using it in other EU countries - that's what is meant by the "no roaming fees" you will read about. This is not the same thing as saying that a SIM card bought in one country will work in another.
The plan you bought in Germany may or may not allow the SIM to be used in the Netherlands, and the German SIM from last year may or may not still be active at all. SIM cards expire; the exact terms of when they expire varies from company to company, and can be just a few weeks after it was last topped up, or can be a year. If the SIM is unexpired and if you have enough credit and if your plan works in Germany, then yes, using the SIM in the Netherlands will cost the same as using it in Germany. But if any of these conditions are not met, you won't be able to use it there.
There's also the issue of refilling; it can be very difficult to refill a SIM card bought in another country (you have to get the website to take your credit card, and they can be very picky about which CC's they accept).
So, the answer is maybe. Since SIMs are so small, just take it, and see when you get there. But, be prepared to buy a new one if it doesn't work, or if it does work but then gets used up before the end of your trip.