MagicFloyd,
since your phone is T-Mobile-Locked, your only chance will be a prepaid T-Mobile SIM. So you need to find a store selling T-Mobile prepaid SIM cards.
I am not sure it'll work in your phone, but titus wong said it worked for him on page 1 of this thread.
For Internet access, I found the following page at t-mobile.nl
http://www.t-mobile.nl/mobiel-internet/ ... der-bundel
again, Google is still fairly bad at translating Dutch,
but here it is
Basically, they're saying that - with a prepaid SIM - you can get 384 kb/s down and 64 kb/s up. In European cell terms that is 3G "basic", without HSDPA. It translates to 30-40 KB/s, which is not great, but sufficient for Google Maps and other stuff to help you around the city.
The price is €1 per MB, but is capped at max. €2,50 per day. So not a bad deal for a visit to Amsterdam.
I have NO experience with T-Mobile.nl, but with your locked phone this is probably not just your best but also your only bet.