Since your saying that you dont have money for more than 1GB right now, im assuming that you mean your temporarily only using 1GB, however you will add another gig once you get more money, correct?
If thats true, then I would go with 1x1GB now, and then add another identical 1GB stick later on. Because if you end up with 4x 512MB sticks, there will be a performance decrease over 2x 1GB sticks.