My local Costco has the Sandisk 4GB Ultra II card on sale for $28. This isn't the cheapest price for a 4GB card by far, but:
1. It has worked perfectly in my MT II. This is really the only reason I have two of them. I'm scared to experiment with other cards, as I've heard some people have had issues with some cards.
2. You can walk into the store and buy it without having to wait for shipping.
--Michael