It's generally not necessary to change your money in St. Maarten/St. Martin. All banks now have ATMs that accept international cards. On the Dutch side, try RBTT or Windward Islands Bank, both of which have several branches on the island. On the French side, try Banque des Antilles Françaises or Banque Française Commerciale, but they will issue only euros. MasterCard, Visa, and American Express are accepted all over the island, Diners Club and Discover on occasion.
Legal tender on the Dutch side is the Netherlands Antilles florin (guilder), written NAf; on the French side, it's the euro (EUR ). The exchange rates fluctuate, but at this writing they were about NAf 1.78 to US$1 and about EUR 1 to US$1.30. On the Dutch side, prices are usually given in both NAf and U.S. dollars, which are accepted all over the island. Legal tender on the French side is the euro, but in practice most places will accept dollars readily and will often give a favorable exchange rate.
Banque des Antilles Françaises (Rue de la République, Marigot. 590/29-13-30). Banque Française Commerciale (Rue de Hollande, Marigot. 590/87-53-80). Credit Mutuel (Rue de la République, Marigot. 590/29-54-90). RBTT (Emnaplein, Philipsburg. 599/542-3344. Union Rd., Cole Bay. 599/544-3078). Windward Islands Bank (Cannegieter St., Philipsburg. 599/542-2313).