A couple of add-ons to what LIH Prem said above:

The "double stays, double points" kicks in after your first stay paid for with a VISA card. If you indeed are going to have a long stay in March, it might be worthwhile to get the initial (qualifying) stay out of the way before you check in for the long stay, so that the long stay will be doubled. You could do that by adding a one-night stay at a Fairfield Inn, or by switching to a Marriott property for something you already have scheduled in another hotel.

Be sure to pay with a VISA card; the "double" promotion doesn't work with other forms of payment. And the "double" promotion ends 31 March.

You might also consider getting the Marriott VISA card immediately, since the card carries a complementary upgrade for Silver for the first year. That way you will get the 20% bonus, and if you use that particular VISA card to pay for your stay, you also get (as mentioned) 3 additional Marriott Rewards points per dollar spent.

(Edited to add) And if your trip is taking you outside the US where your charge will be in foreign currency, the Marriott VISA card only charges a 1% currency exchange fee rather than the more usual 3% charge - but that changes 1 April 2002, when they raise the currency conversion charge to 3% on the Marriott VISA, too.

[This message has been edited by Counsellor (edited 03-02-2002).]